I have the "Check for updates, but let me choose whether to install them" option selected. Usually I update hoping that bugs that freeze or slow down the browser will be ironed out and the new version will be streamlined. The newest version is really awful with less functionality than previously. Mozilla seem to be going for a flash, Chrome look alike appearance with all sorts of fancy roundy transparent tabs and tiny toolbar buttons instead of a simple user interface. The new bookmark display is almost as bad as the expanding menu system used in Chrome. It's possible to pin bookmarks as a sidebar, but when history is selected, it expands out again and has to be re-pinned.
Lots of people over on the Mozilla support forum are going mad over this update!
If you want to downgrade, this is the link to the Mozilla support page, which links to the FTP server with previous versions.
Version 28 final is also available on the MajorGeeks.com site.