First thing to do when you get a browser problem is to delete cache and cookies, close all browser windows and try again. Also try to disable any plugins you might have installed and see if it helps (if it does, get rid of the plugin that is causing troubles).
Running a Malwarebytes scan would also be a good idea.
If none of the above work, uninstall and reinstall the browser. If that doesn't work either, backup your bookmarks (important if you want to keep them), uninstall again and delete the Firefox app data folder, then reinstall.