Be sure to delete the Firefox program folder before reinstalling Firefox.
(32 bit Windows) C:\Program Files\Mozilla Firefox\
(64 bit Windows) C:\Program Files (x86)\Mozilla Firefox\
Your bookmarks and other profile data are stored in the Firefox Profile Folder and won't be affected by an uninstall and (re)install, but make sure that "remove personal data" is NOT selected when you uninstall Firefox.
If you keep having problems then also create a new profile.