From MozillaZine Knowledge Base
This article deals with text overlapping on Mac OS X, where text in web pages, text boxes or e-mail messages becomes jumbled. The problem may be caused by duplicate fonts. To fix it:
- In the Applications Folder, open Font Book
- In the "Collection" column, select "Computer"
- All duplicates will have a bullet after them. Use the command key to select all of the fonts with duplicates
- Choose Edit -> Resolve duplicates
- Restart Firefox/Thunderbird/whatever