In Settings Menu of Firefox browser what do you see in the first section, do you see the message '''Firefox is currently your default browser''' ? Refer to the attached screenshot for more details on how it should be if it has been default for years. If it is not then please make it so. Also If you are on Windows go to '''Settings > Apps > Default Apps''' and check what is set in the '''Web Browser''' section. Is it Firefox or something else? If it is something else then change it to Firefox.