Text overlaps on Mac

This article deals with text overlapping on Mac OS X, where text in text boxes or e-mail messages becomes more jumbled the longer lines become. The problem may be caused by duplicate fonts. To fix it:

1. In the Applications Folder, open Font Book
2. In the "Collection" column, select "Computer"
3. All duplicates will have a bullet after them. Use the command key to select all of the fonts with duplicates
4. Choose Edit -> Resolve duplicates
5. Restart Firefox/Thunderbird/whatever