I recently had a funky thing happen to my Firefox (I am running FF 2.0 and Mac OS X Tiger). I would visit webpages and the text would be smashed and overlapping. I couldn’t read certain words and highlighting text to copy was a pain. I turned off the CSS in the browser. The text looked fine. I then started Firefox in safe mode (no add -ons running). I still had overlapping text. I found a blog that had the solution posted for my overlapping text issue (http://www.pixelitdigital.com/?p=45). The issue was having duplicate fonts on different parts of my computer. Here is a run down of how I cleaned up my fonts and fixed the overlapping and smashed text issue:
3. Click and select all the fonts that have a dot next to them.
4. In the menu bar, select Edit, then click on Resolve Duplicates.
(The author in the article above indicates you might need find the duplicate and remove the font by hand, but I did not have to).
Restart Firefox and type away without overlapping text. Enjoy!
Note: If these set of directions do not solve the issue, then go into the Firefox preferences, choose the Content tab, and change your Default to something like Tahoma.