Well, just tried building Firefox with GCC 9.4.0, IIRC, gcc-9.4.0-os2-20220314b.zip, and it succeeded.
about:buildconfig
Source
Built from git commit 90c195f3e9a9219bc7e264ee1a98eab88d8af903
Build platform
target
i386-pc-os2-emx
Build tools
Compiler Version Compiler flags
gcc.exe 9.4.0 -Wall -Wempty-body -Wpointer-to-int-cast -Wsign-compare -Wtype-limits -Wno-unused -Wcast-align -Zomf -std=gnu99 -fgnu89-inline -fno-strict-aliasing -Zomf -fno-math-errno -pthread
c++ 9.4.0 -idirafter K:/usr/include/os2tk45 -Wall -Wempty-body -Woverloaded-virtual -Wsign-compare -Wwrite-strings -Wno-invalid-offsetof -Wcast-align -Zomf -fno-exceptions -fno-strict-aliasing -Zomf -flifetime-dse=1 -fno-delete-null-pointer-checks -Wclass-memaccess -fno-rtti -fno-exceptions -fno-math-errno -std=gnu++0x -pthread -DNDEBUG -DTRIMMED -g -march=pentium4 -mtune=generic -O2 -flifetime-dse=1 -fno-delete-null-pointer-checks -Wclass-memaccess -msse2 -fomit-frame-pointer
Configure arguments
--enable-application=browser '--enable-optimize=-march=pentium4 -mtune=generic -O2 -flifetime-dse=1 -fno-delete-null-pointer-checks -Wclass-memaccess -msse2' --disable-debug --enable-debug-symbols --disable-tests --enable-os2-high-mem --without-x --enable-official-branding --enable-system-cairo --enable-system-pixman --with-intl-api --with-system-icu --with-system-zlib --with-system-libvpx --prefix=/usr --disable-webrtc --disable-telemetry --disable-devtools --disable-crashreporter --disable-gamepad --disable-web-speech --disable-updater --disable-startupcache
Guess I'll try SeaMonkey, then download 10.x and try that.