Firefox will try to keep your bookmarks etc. It may be a good practice to somehow back up your bookmarks on a regular basis, maybe by doing a manual backup

• see backing up and restoring bookmarks

Firefox tries to keep the program Files separate from most of the settings etc which will be in the profile. If you do uninstall Firefox do NOT use any option that says remove personal settings, however that is worded. Such options attempt to remove all profile data, bookmarks passwords etc from all normally installed versions of firefox.

Sometimes a clean re-install is required, if so take care that you delete the program files not the profile files

• see http://kb.mozillazine.org/Standard_diagnostic_%28Firefox%29#Clean_reinstall

Be aware that reinstalling Firefox won't fix issues caused by a corrupted or otherwise malfunctioning profile.

What kind of problems do you have?

• https://support.mozilla.com/kb/Basic+Troubleshooting