Missing scroll bars
Recently I updated and noticed to my shock that all the scrollbars had gone. Which wasn't very helpful. I finally found a bug report for it (#305391), but this put the blame, once again, on the theme makers.
Luckily the reason behind this was an improvement to cross platform compatibility on themes (bug #222654). Although this doesn't help me, as finding theme updates is really rather difficult.
Luckily there is a quick way of fixing it yourself. This is how.
1. Navigate to your user profile directory for Mozilla, and locate the latest theme *.JAR file for it.
2. Extract this (keeping the directory tree) with WinZip (or something).
3. In the General directory copy the file scrollbars.css to xulscrollbars.css and nativescrollbars.css.
4. Re-compress the directory tree in the same way and replace the original JAR file (keep the same name).
5. Restart Mozilla.
6. Tada!
Hope this helps somebody.