Hello,
I have observed a crash in firefox with glibc-2.34 and have found a
small reproducer.
Is the sigsys signal handler valid? If yes, then there is a bug in
glibc-2.34.
If it is invalid to set the result in the context, I think the firefox
sandbox is doing dodgy things.

gcc test.c -lseccomp
strace ./a.out

This test reproducer does not segfault with 2.33 (it gives a
"Permission denied")

Cheers
Andreas