Reply To: Firefox ESR updates (91.4, Oct ESR 102.3)

Confusion here by many, including me.

I didn’t know firefox-esr 78.15 (built by DEBIAN) still supported non-sse2 processors, so I am sorry for the confusion. Official support was dropped by Mozilla, but it seems Debian still built it with sse support.

Firefox built by MX is the NON-ESR version, which I doubt includes non-sse2 support, and is available in the antiX repos.
It seems that firefox-esr used a fallback for non-sse2 support

  * js/src/jit/BaselineJIT.h: Disable baseline JIT when SSE2 is not supported in firefox 62
    at runtime. bz#1492064. Closes: #908396, #908449.
  * gfx/2d/Swizzle.cpp: Use Swizzle fallback when SSE2 is not supported.
    bz#1492065. Closes: #877445.

But I cannot find if this was still used in newer firefox-esr 78.

Maybe you need to disable some flags in about:config. I will see if I can find any issue related to this, but initial search results aren’t very clear.