SSE2

SSE2 instruction set support requires a either Pentium 4, Netburst based Celeron, Athlon 64, Via C7 or newer. Firefox after version 48 requires it. New Moon and K-meleon offer builds that don’t require it.

SSE3
The next level of SSE is staring to become mandatory for some newer applications but they mostly don’t support xp anyway. It likely affects Windows 7 users though.