Mozilla lists the minimum requirements for Firefox 144 as “1 gigahertz (GHz) or faster compatible processor or System on a Chip (SoC)” and 1 GB of RAM.

• 32-bit Pentium III and 4 will support up to 4 GB of RAM, but it’s going to be dogshit slow.
• 1 GHz is clearly a simplification for a more complex metric and very obviously doesn’t have in mind a Pentium III 1400 @ 1.4 GHz.

As for Pentium 4, you need to qualify that, because Pentium 4 wasn’t exclusively 32-bit – only the earlier ones were. Cedar Mill and many Prescotts supported Intel 64. So we’ll assume a generous case that someone is using something like a Pentium 4 505, where “capable of running” probably still isn’t the same as “running decently on any modern website”.

• macOS 10.15 required for Firefox 144, so no macOS users.
• Windows 10 is EOL, and even then, 22H2 doesn’t officially support it.
  • Firefox 144 does not support Windows 7, 8, or 8.1.
  • Windows 11 straight-up will not boot with that CPU.

So you’re left with Linux or BSD. Firefox’s Linux requirements are fairly lax, so we can assume you’re not running into compat issues. That leaves you with someone who:

1. Is using a CPU manufactured around 2004 or earlier.
2. Is on Windows 10 32-bit (despite EOL) Linux, or BSD.
3. Specifically wants to use Firefox.

That’s such an obscenely negligible percentage of people (and will keep falling off a cliff as that hardware dies out and fewer applications support it) that, combined with the terrible UX of a modern web browser on that hardware, the baggage that comes with not assuming 64-bit can’t be justified.

Looks like the Pi 1, 2, and Zero are 32-bit (and probably other old or low spec SBCs). I’m also not sure if Firefox is distinguishing between 32-bit ARM and 32-bit x86.

good grief