Actually, firefox-esr doesn't even work on real x86_32 CPUs (except Pentium 4) since years. It requires SSE2 without the sse2-support package, so basically it is committing a baseline violation on i386. Currently, due to some random GCC behavior, it doesn't crash instantly when I try to open any page. But this can change in any upcoming versions.
There is a fix, but it is not merged since months: https://salsa.debian.org/mozilla-team/firefox/-/merge_requests/7 If it was merged, it would close the following bugs: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=923785 https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=961663 https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1009808 https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1002600 https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1019246 Alternatively, epiphany-browser could be used instead of firefox-esr on i386, because that works without SSE or even without MMX.
