Start Firefox in Safe Mode to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance).

• Do not click the Reset button on the Safe mode start window or otherwise make changes.

• https://support.mozilla.org/kb/Safe+Mode

Try to disable hardware acceleration in Firefox.

• Tools > Options > Advanced > General > Browsing: "Use hardware acceleration when available"
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

• https://hacks.mozilla.org/2010/09/hardware-acceleration/

Disabling Hardware acceleration did it. That surprises me with my computer's specs. What do I have to update to be able to use the hardware acceleration?

Win 7 Ultimate, 64 bit ------ Intel i7 Lynnfield CPU -------- 16 Gigs RAM ----------- EVGA P55 FTW 200 mobo ----------- EVGA GTX 570 GPU -------

If hardware acceleration is causing problems then that means that Firefox can't use the GPU of the your graphics display card to unload the main CPU and perform some actions faster.
You may not notice any difference is most cases. The only solution is to check if there is an update of the driver of your graphics display card.

• https://support.mozilla.org/kb/how-do-i-upgrade-my-graphics-drivers

Thanks!