After upgrading to Tiger, I had awful problems with both Mozilla Thunderbird and Mozilla Firefox. In ThunderBird, HTML composition was awful -- typing would get ahead of the text displayed until you hit enter, and if you backspaced, forget trying to decide what was there. In Firefox, the problem manifested itself as a text kerning/rendering issue. Links or bolded fonts would sometimes overlay the word next to them that was in normal font.
I figured out what was causing this, at least for me. For some reason, I had a full copy of both the FontCollections and Fonts folders from /System/Library in my user's Library. I have no idea how they got there, but I did the following to get rid of them:
$ cd ~/Library
$ tar cf ~/LibraryUserFonts.tar FontCollections Fonts
$ rm -rf FontCollections/*
$ rm -rf Fonts/*
Log out and log back in. The display problems in both FireFox and ThunderBird have now completely disappeared.
[robg adds: This is a fairly specific incident, but since the author doesn't know how the copies of the fonts wound up in the local Library, I thought it might be worth running, just in case someone else is having similar issues.]

