Reset the page zoom on pages that cause problems: View > Zoom > Reset (Ctrl+0 (zero); Cmd+0 on Mac)

• http://kb.mozillazine.org/Zoom_text_of_web_pages

In Firefox 4 Safe mode disables extensions and disables hardware acceleration.

• Tools > Options > Advanced > General > Browsing: "Use hardware acceleration when available"

If disabling hardware acceleration works then check if there is an update available for your graphics display driver.

Thx! Disable hardware acceleration works for me but there's no update available for my graphics display driver.

You can try if it works with hardware acceleration enabled, but with some other features disabled.

Try to toggle some Boolean gfx.font_rendering prefs on the about:config page to disable some features and maybe set gfx.direct2d.disabled -> true to disable Direct2D.
Filter: gfx

To open the about:config page, type about:config in the location (address) bar and press the "Enter" key, just like you type the url of a website to open a website.
If you see a warning then you can confirm that you want to access that page.
You can use the Filter bar at to top of the about:config page to locate a pref more easily.

Great find, setting gfx.direct2d.disabled to true (thus disabling it) has solved my issue. Thank you very much!

I'm curious though, isn't this just another way of turning off hardware acceleration?

Yes, you are turning off hardware acceleration for rendering fonts this way.
There may be other tweaks possible, but you would need advice from someone who fully understands how all the prefs work.

You can try:

• Anti-Aliasing Tuner: https://addons.mozilla.org/firefox/addon/anti-aliasing-tuner/