From Change in system requirements:
Additional information in the referenced article.
QuoteWith the next development release (33.3.0) we will be changing Pale Moon's system requirements.
Starting that version, all 64-bit release versions will require at least the AVX instruction set in the CPU. This instruction set has been common in processors for about 10 years.
This follows up on our announcements earlier this year where the exact time this change would be made was not yet set.
In tandem with this change, the expected Linux build environments will also need to be changed to start using gcc v11 or later for official builds, as we will be setting x86_64 v2 + avx as our expected target architecture which cannot be properly done on earlier versions of gcc.
Windows 32-bit builds will not be changing their requirements, and continue to use a minimum of SSE2.
Additional information in the referenced article.