When you have a slow PC, you have to disable hardware acceleration (in Firefox) so the acceleration doesn't slow your browser down, and if you don't see a difference between on and off.. then there's less reason to lose sleep about it.
Well, perhaps it's the whole idea of using a GTK3, 3D accelerated desktop be it Cinnamon or Gnome 3 or other that is stupid. You copy data into buffers and call some OpenGL functions, bloated rendering stack and buggy driver, instead of just displaying shit like is done in Mate, Xfce or Ice WM etc.
You waste a ton of CPU, but call that 3D "accelerated". But if you have a fast CPU (e.g. dual core / quad thread Intel at 2.5 GHz) and run Firefox, Chrome, Libre Office then it's plenty ok.