Another difference in Safe Mode is that hardware acceleration of graphics is disabled. Since that feature is useful for the appearance of text (smoother anti-aliasing), it would be unfortunate if you had to disable it, but do you want to test that anyway so we could pin the blame on Firefox being incompatible with your graphics card/chipset driver software? Here's how:

"3-bar" menu button (or Tools menu) > Options

In the left column, click Advanced. On the right side, with the "General" mini-tab active, uncheck the box for "Use hardware acceleration when available".

This takes effect the next time you exit Firefox and start it up again. Any difference?

If that helps, you may want to check whether graphics card/chipset driver updates are available for your system. This article suggests safe ways to do that: Upgrade your graphics drivers to use hardware acceleration and WebGL.