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Build log:
http://download.eng.bos.redhat.com/brewroot/work/tasks/2535/23602535/build.log

devtoolset-8-gcc-c++-8.3.1-3.1.el6.x86_64
devtoolset-8-gcc-8.3.1-3.1.el6.x86_64
devtoolset-8-binutils-2.30-54.el6.x86_64
devtoolset-8-libatomic-devel-8.3.1-3.1.el6.x86_64

Firefox build fails with:

11:16.73 {standard input}: Assembler messages:
11:16.75 {standard input}:59668: Warning: end of file not at end of a line; newline inserted
11:16.75 {standard input}:60789: Error: unknown pseudo-op: `.lbb1'
11:16.75 {standard input}: Error: open CFI at the end of file; missing .cfi_endproc directive
11:16.76 g++: fatal error: Killed signal terminated program cc1plus

This is strange, I've never seen that `.lbb1' and can't find it anywhere, either.  Are you sure you didn't just run out of memory?  I.e., try to build Firefox again, this time on a box with more RAM.

I restricted number of parallel builds to 2 and it went through. I'll watch that but looks like it's really OOM.

The assembler warnings/errors are just due to -pipe and cc1plus being killed by the kernel, while the assembler is parsing what it produces.