Parece ser esse problema conhecido: link
Citando as respostas:
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/
Try to set the gfx.content.azure.enabled pref to false or if this didn't help disable Direct2D by setting the gfx.direct2d.disabled pref to true on the about:config page and leave hardware acceleration otherwise enabled.
Alternativamente:
The actual problem (atleast for me and a few others) is due to a windows update kb2670838. This update applies performance patches to Direct2D, 3D, and Write related components. The reason disabling usage of Direct2D or hardware acceleration fixes it is because its simply not using those Direct___ functions anymore.
In the Update history, 2670838 is listed as: Platform update for Microsoft Windows (KB2670838) In the Installed updates, 2670838 is listed as: Update for Microsoft Windows (KB2670838)
Again, uninstalling this update solved the problem for me and a few others, hopefully it will solve your problems correctly as well.