Some of the things not on as default in Firefox that make it work better are:

layout.spellcheckDefault

Set this to 2, instead of 1 - as that makes it check any single line areas (titles on forums, etc), instead of only multiline areas with 1, and 0 is off.


gfx.webrender.all

This makes it that the graphics card will process the graphics, rather than the processor.

If you have a dual graphic system, also make sure to go into Windows Settings - Display - Graphics Settings - Graphics Performance Settings - then make Windows use the High Performance Graphics card for Firefox.