Clear the cache and the cookies from sites that cause problems.

"Clear the Cache":

• Firefox/Tools > Options > Advanced > Network > Cached Web Content: "Clear Now"

"Remove Cookies" from sites causing problems:

• Firefox/Tools > Options > Privacy > Cookies: "Show Cookies"

You can remove all data stored in Firefox from a specific domain via "Forget About This Site" in the right-click context menu of an history entry ("History > Show All History" or "View > Sidebar > History") or via the about:permissions page.

Using "Forget About This Site" will remove all data stored in Firefox from that domain like bookmarks, cookies, passwords, cache, history, and exceptions, so be cautious and if you have a password or other data from that domain that you do not want to lose then make a note of those passwords and bookmarks.

You can't recover from this 'forget' unless you have a backup of the affected files.

It doesn't have any lasting effect, so if you revisit such a 'forgotten' website then data from that website will be saved once again.

Hi!

Uninstalling and reinstalling Firefox doesn't, unfortunately, remove your information and possible corrupted data from your computer.

In these cases we recommend using Firefox Reset to clean all the possible errors in your Firefox profile so it behaves as a clean installation. Check this article that explains how to use the feature: Refresh Firefox - reset add-ons and settings

Let me know if this helps. Ibai