Could you clarify:

Is the problem with your Firefox Account: you are attempting to recover data by setting up Sync using your Firefox Account passwords?

Is the problem with local files that the Firefox installer found after your reinstall, but now cannot be accessed without a Master Password? This password would need to have been entered every time you restarted Firefox, so it seems unlikely you would have forgotten this one.

If you did an uninstall-reinstall without intentionally removing your Firefox data, check out:

Profile Manager Page

Inside Firefox, type or paste about:profiles in the address bar and press Enter/Return to load it.

This page should list at least one profile and could list many. (Each profile has two folders and one or more buttons. Do not use any Remove buttons!)

The profile that Firefox is currently using will have this:

This is the profile in use and it cannot be deleted.

Do you have another profile named default or default-longnumber listed on this page? If so, you can check whether it is the one you were using recently by clicking its Launch profile in new browser button.

If it isn't what you want, simply close that new window.

If it IS what you want, back on the about:profiles page, click the Set as default profile button for that profile so Firefox uses it automatically at the next startup.

Note: Before closing the page, double-check that the default profile is set to what you want, in case viewing them temporarily made one the default.

That fixed it, I can't thank you enough!!!!!!!

Has been a very frustrating day until now. I am so relieved, can now get back to normal.

Thank you very much!!!!