Last versions of Firefox to support each version of Windows:

Last to support Windows 7 & 8.1: 115.19.0 (ESR) Source
Windows 8 (2012) might work with this through Windows 7 Backwards capability
Supports an CPU as old as a Pentium 4 (Not older due to SSE2 needed for x86 CPU's)
Released: January 7, 2025 Source

Last to support Windows XP & Vista: 52.9.0 (ESR) Source 1, Source 2
Supports an CPU as old as a Pentium 4 (Not older due to SSE2 needed for x86 CPU's)
Released: June 26, 2018 Source

Last to support Windows 2000: 12.0 Source
Supports an CPU as old as a Pentium 4 (Not older due to SSE2 needed for x86 CPU's)
No sub updates (besides beta's) for version 12.
Released: April 24, 2012 Source

Last to support Windows NT4/98/98SE/ME: 2.0.0.20 Source
Supports an CPU as old as a Intel Pentium II.
Released: December 18, 2008 Source

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