I added an 8GB swapfile, and (slowly) got this backtrace:
# cat error.txt
Thread 1 "firefox" received signal SIGSEGV, Segmentation fault.
i32_load8_u (addr=2014643200, mem=<optimized out>) at rlbox.wasm.c:146
146 rlbox.wasm.c: No such file or directory.
(gdb) bt
#0 i32_load8_u (addr=2014643200, mem=<optimized out>) at rlbox.wasm.c:146
#1 w2c_rlbox_streqci
(var_p0=***@entry=262000, var_p1=2014643200, instance=<optimized out>)
at rlbox.wasm.c:774
#2 0x00007fffe9195c48 in w2c_rlbox_getEncodingIndex
(var_p0=<optimized out>, instance=<optimized out>) at rlbox.wasm.c:66243
#3 w2c_rlbox_getEncodingIndex (var_p0=262000, instance=0x7fffd992c000)
at rlbox.wasm.c:690
#4 w2c_rlbox_MOZ_XmlInitEncodingNS_0
(instance=0x7fffd992c000, var_p0=438820, var_p1=438816, var_p2=262000)
at rlbox.wasm.c:2316
#5 0x00007fffe91e305c in w2c_rlbox_initializeEncoding
(instance=***@entry=0x7fffd992c000, var_p0=***@entry=438672)
at rlbox.wasm.c:48270
#6 0x00007fffe92a77a8 in w2c_rlbox_prologInitProcessor
(instance=0x7fffd992c000, var_p0=438672, var_p1=445856, var_p2=511392, var_p3=262140) at rlbox.wasm.c:45278
#7 0x00007fffe92b82e4 in w2c_rlbox_MOZ_XML_Parse_0
(var_p3=0, var_p2=65536, var_p1=445856, var_p0=438672, instance=0x7fffd992c000) at rlbox.wasm.c:49293
#8 w2c_rlbox_MOZ_XML_Parse
(instance=0x7fffd992c000, var_p0=438672, var_p1=445856, var_p2=65536, var_p3=0) at rlbox.wasm.c:20709
--Type <RET> for more, q to quit, c to continue without paging--
#9 0x00007fffea00fd50 in rlbox::rlbox_wasm2c_sandbox::impl_invoke_with_func_ptr<XML_Status (XML_ParserStruct*, char const*, int, int), XML_Status (unsigned int, unsigned int, int, int), unsigned int, unsigned int, unsigned int, bool>(XML_Status (*)(unsigned int, unsigned int, int, int), unsigned int&&, unsigned int&&, unsigned int&&, bool&&) (func_ptr=<optimized out>, this=0x7fffd992c000)
at ./build-browser/dist/include/mozilla/rlbox/rlbox_wasm2c_sandbox.hpp:763
#10 rlbox::rlbox_sandbox<rlbox::rlbox_wasm2c_sandbox>::INTERNAL_invoke_with_func_ptr<XML_Status (XML_ParserStruct*, char const*, int, int), rlbox::tainted<XML_ParserStruct*, rlbox::rlbox_wasm2c_sandbox>&, rlbox::tainted<char const*, rlbox::rlbox_wasm2c_sandbox>, unsigned long, bool>(char const*, void*, rlbox::tainted<XML_ParserStruct*, rlbox::rlbox_wasm2c_sandbox>&, rlbox::tainted<char const*, rlbox::rlbox_wasm2c_sandbox>&&, unsigned long&&, bool&&) (func_name=<optimized out>, func_ptr=<optimized out>, this=<optimized out>)
at ./build-browser/dist/include/mozilla/rlbox/rlbox_sandbox.hpp:790
#11 nsExpatDriver::ParseChunk(char16_t const*, unsigned int, nsExpatDriver::ChunkOrBufferIsFinal, unsigned int*, unsigned long*)
(this=0x7ffff4856400, aBuffer=<optimized out>, aLength=***@entry=32768, aIsFinal=***@entry=nsExpatDriver::ChunkOrBufferIsFinal::FinalChunk, aConsumed=0x7fffffffc50c, aLastLineLength=0x7fffffffc510) at ./parser/htmlparser/nsExpatDriver.cpp:1248
#12 0x00007fffea01015c in nsExpatDriver::ChunkAndParseBuffer(char16_t const*, unsigned int, bool, unsigned int*, unsigned int*, unsigned long*)
(this=***@entry=0x7ffff4856400, aBuffer=***@entry=0x7fffd8f90020 u"<?xml version=\"1.0\"?>\n\n<!-- The \"global.css\" stylesheet is imported first to allow other stylesheets to\n override rules using selectors with the same specificity. This applies to\n both \"conten"..., aLength=***@entry=32768, aIsFinal=***@entry=false, aPassedToExpat=***@entry=0x7fffffffc508, aConsumed=***@entry=0x7fffffffc50c, aLastLineLength=***@entry=0x7fffffffc510) at ./parser/htmlparser/nsExpatDriver.cpp:1204
#13 0x00007fffea0105a0 in nsExpatDriver::ResumeParse(nsScanner&, bool)
(this=0x7ffff4856400, aScanner=..., aIsFinalChunk=***@entry=false) at ./parser/htmlparser/nsExpatDriver.cpp:1352
#14 0x00007fffea016e3c in nsParser::ResumeParse(bool, bool, bool)
(this=***@entry=0x7fffd99a2260, allowIteration=***@entry=true, aIsFinalChunk=***@entry=false, aCanInterrupt=***@entry=true) at ./parser/htmlparser/nsParser.cpp:715
#15 0x00007fffea017d20 in nsParser::OnDataAvailable(nsIRequest*, nsIInputStream*, unsigned long, unsigned int)
(sourceOffset=<optimized out>, aLength=<optimized out>, pIStream=<optimized out>, request=<optimized out>, this=0x7fffd99a2260)
at ./parser/htmlparser/nsParser.cpp:1027
#16 nsParser::OnDataAvailable(nsIRequest*, nsIInputStream*, unsigned long, unsigned int)
(this=0x7fffd99a2260, request=<optimized out>, pIStream=<optimized out>, sourceOffset=<optimized out>, aLength=<optimized out>)
at ./parser/htmlparser/nsParser.cpp:978
#17 0x00007fffe9fd90a8 in .non-virtual thunk to mozilla::parser::PrototypeDocumentParser::OnDataAvailable(nsIRequest*, nsIInputStream*, unsigned long, unsigned int) () at ./parser/prototype/PrototypeDocumentParser.h:42
#18 0x00007fffe9f4bf30 in nsDocumentOpenInfo::OnDataAvailable(nsIRequest*, nsIInputStream*, unsigned long, unsigned int)
--Type <RET> for more, q to quit, c to continue without paging--c
(this=<optimized out>, request=<optimized out>, inStr=<optimized out>, sourceOffset=<optimized out>, count=<optimized out>)
at ./uriloader/base/nsURILoader.cpp:202
#19 0x00007fffe9c18610 in mozilla::net::ParentChannelWrapper::OnDataAvailable(nsIRequest*, nsIInputStream*, unsigned long, unsigned int)
(this=<optimized out>, aRequest=<optimized out>, aInputStream=<optimized out>, aOffset=<optimized out>, aCount=<optimized out>)
at ./netwerk/ipc/ParentChannelWrapper.h:27
#20 0x00007fffe9ab4550 in mozilla::net::ParentChannelListener::OnDataAvailable(nsIRequest*, nsIInputStream*, unsigned long, unsigned int)
(this=0x7fffdfce9d00, aRequest=0x7ffff48b4dc0, aInputStream=0x7ffff73c93e0, aOffset=0, aCount=<optimized out>)
at ./netwerk/protocol/http/ParentChannelListener.cpp:115
#21 0x00007fffe9f4bf30 in nsDocumentOpenInfo::OnDataAvailable(nsIRequest*, nsIInputStream*, unsigned long, unsigned int)
(this=<optimized out>, request=<optimized out>, inStr=<optimized out>, sourceOffset=<optimized out>, count=<optimized out>)
at ./uriloader/base/nsURILoader.cpp:202
#22 0x00007fffe9ed83d4 in nsJARChannel::OnDataAvailable(nsIRequest*, nsIInputStream*, unsigned long, unsigned int)
(this=0x7ffff48b4dc0, req=<optimized out>, stream=0x7ffff73c93e0, offset=0, count=288947) at ./modules/libjar/nsJARChannel.cpp:1312
#23 0x00007fffe958f858 in nsInputStreamPump::OnStateTransfer() (this=***@entry=0x7ffff4852080) at ./netwerk/base/nsInputStreamPump.cpp:584
#24 0x00007fffe958fde8 in nsInputStreamPump::OnInputStreamReady(nsIAsyncInputStream*) (this=0x7ffff4852080, stream=<optimized out>)
at ./netwerk/base/nsInputStreamPump.cpp:411
#25 0x00007fffe9377314 in CallbackHolder::CallbackHolder(nsIAsyncInputStream*, nsIInputStreamCallback*, unsigned int, nsIEventTarget*)::{lambda()#1}::operator()() const (__closure=<optimized out>) at ./xpcom/io/nsPipe3.cpp:73
#26 NS_NewCancelableRunnableFunction<CallbackHolder::CallbackHolder(nsIAsyncInputStream*, nsIInputStreamCallback*, unsigned int, nsIEventTarget*)::{lambda()#1}>(char const*, CallbackHolder::CallbackHolder(nsIAsyncInputStream*, nsIInputStreamCallback*, unsigned int, nsIEventTarget*)::{lambda()#1}&&)::FuncCancelableRunnable::Run() (this=<optimized out>) at ./build-browser/dist/include/nsThreadUtils.h:667
#27 NS_NewCancelableRunnableFunction<CallbackHolder::CallbackHolder(nsIAsyncInputStream*, nsIInputStreamCallback*, unsigned int, nsIEventTarget*)::{lambda()#1}>(char const*, CallbackHolder::CallbackHolder(nsIAsyncInputStream*, nsIInputStreamCallback*, unsigned int, nsIEventTarget*)::{lambda()#1}&&)::FuncCancelableRunnable::Run() (this=<optimized out>) at ./build-browser/dist/include/nsThreadUtils.h:665
#28 0x00007fffe93d8440 in mozilla::RunnableTask::Run() (this=0x7fffe6925000) at ./xpcom/threads/TaskController.cpp:559
#29 mozilla::TaskController::DoExecuteNextTaskOnlyMainThreadInternal(mozilla::detail::BaseAutoLock<mozilla::Mutex&> const&)
(this=***@entry=0x7ffff4841a00, aProofOfLock=...) at ./xpcom/threads/TaskController.cpp:886
#30 0x00007fffe93d9144 in mozilla::TaskController::ExecuteNextTaskOnlyMainThreadInternal(mozilla::detail::BaseAutoLock<mozilla::Mutex&> const&) (this=***@entry=0x7ffff4841a00, aProofOfLock=...) at ./xpcom/threads/TaskController.cpp:709
#31 0x00007fffe93d9614 in mozilla::TaskController::ProcessPendingMTTask(bool) (aMayWait=false, this=0x7ffff4841a00)
at ./xpcom/threads/TaskController.cpp:495
#32 operator() (__closure=<optimized out>) at ./xpcom/threads/TaskController.cpp:218
#33 mozilla::detail::RunnableFunction<mozilla::TaskController::TaskController()::<lambda()> >::Run(void) (this=<optimized out>)
at ./xpcom/threads/nsThreadUtils.h:548
#34 0x00007fffe93f284c in nsThread::ProcessNextEvent(bool, bool*) (this=0x7ffff48b0300, aMayWait=<optimized out>, aResult=0x7fffffffd1b7)
at ./xpcom/threads/nsThread.cpp:1199
#35 0x00007fffe93f5954 in NS_ProcessNextEvent(nsIThread*, bool) (aThread=<optimized out>, aMayWait=<optimized out>)
at ./xpcom/threads/nsThreadUtils.cpp:480
#36 0x00007fffe9d6736c in mozilla::ipc::MessagePump::Run(base::MessagePump::Delegate*) (this=0x7fffe8554700, aDelegate=0x7ffff7381c80)
at ./ipc/glue/MessagePump.cpp:85
#37 0x00007fffe9d070e4 in MessageLoop::RunInternal() (this=<optimized out>) at ./build-browser/dist/include/mozilla/RefPtr.h:319
#38 MessageLoop::RunHandler() (this=<optimized out>) at ./ipc/chromium/src/base/message_loop.cc:363
#39 MessageLoop::Run() (this=<optimized out>) at ./ipc/chromium/src/base/message_loop.cc:345
#40 0x00007fffececdfa8 in nsBaseAppShell::Run() (this=0x7fffe85db780) at ./widget/nsBaseAppShell.cpp:148
#41 0x00007fffee34a918 in nsAppStartup::Run() (this=0x7fffe8584840) at ./build-browser/dist/include/nsCOMPtr.h:747
#42 0x00007fffee491114 in XREMain::XRE_mainRun() (this=***@entry=0x7fffffffd798) at ./toolkit/xre/nsAppRunner.cpp:5683
#43 0x00007fffee491fa8 in XREMain::XRE_main(int, char**, mozilla::BootstrapConfig const&)
(this=***@entry=0x7fffffffd798, argc=***@entry=1, argv=***@entry=0x7fffffffefa8, aConfig=...) at ./toolkit/xre/nsAppRunner.cpp:5884
#44 0x00007fffee4927ec in XRE_main(int, char**, mozilla::BootstrapConfig const&) (argc=1, argv=0x7fffffffefa8, aConfig=...)
at ./toolkit/xre/nsAppRunner.cpp:5940
#45 0x00007fffee4942cc in mozilla::BootstrapImpl::XRE_main(int, char**, mozilla::BootstrapConfig const&)
(this=<optimized out>, argc=<optimized out>, argv=<optimized out>, aConfig=<optimized out>) at ./toolkit/xre/Bootstrap.cpp:45
#46 0x00000001000092b0 in do_main(int, char**, char**) (argc=<optimized out>, argv=<optimized out>, envp=***@entry=0x7fffffffefb8)
at ./browser/app/nsBrowserApp.cpp:227
#47 0x000000010000814c in main(int, char**, char**) (argc=<optimized out>, argv=<optimized out>, envp=0x7fffffffefb8)
at ./browser/app/nsBrowserApp.cpp:445
(gdb)