Are you using some custom CSS code or an extension to move the position of your address bar? The window controls for closing, minimizing etc. are not on the same level as the address bar by default. Those customization could be the source of your problem. I would recommend temporarily starting Firefox in [[Use Troubleshoot Mode in Firefox|troubleshoot mode]] to see if that resolves the text size issue. If it doesn't, you can close out of troubleshoot mode without making any permanent modifications. If it does, considering [[Refresh Firefox - reset add-ons and settings|refreshing Firefox]] to resolve the issue permanently.