my browser say's it's corrupt

Copy your Profile before you reinstall, if you go down that path

In Windows it's C:\Users\~~~name---\AppData\Roaming\Mozilla\Firefox\Profiles\~???##.default=release (or some other, you may have two.

That profile has all your bookmarks, passwords, site settings etc.

Note that the user is on Mac OSX 10.9 and can only run Firefox 78.15.0 ESR as the last supported version.

Can you attach a screenshot?

• https://support.mozilla.org/en-US/kb/how-do-i-create-screenshot-my-problem
• use a compressed image type like PNG or JPG to save the screenshot

Firefox 78.15.0 ESR is the last version for Mac OS X 10.9/10.10/10.11 (distributionID: mozilla-mac-eol-esr1) and you will no longer receive updates.

• https://support.mozilla.org/en-US/kb/firefox-mac-osx-users-esr

Firefox 78.15.0 on the Mozilla CDN server:

• https://download-installer.cdn.mozilla.net/pub/firefox/releases/78.15.0esr/