I think you are referring to the '''about:config''' page. Do you recall what kind of changes you made? For example, did you modify the value of the '''layout.css.devPixelsPerPx''' or '''ui.textScaleFactor''' preference? Is Firefox still open or did you close it? If Firefox is still open, you may be able to fix this using the same page, but you will need to zoom it significantly (hold down Ctrl and tap the + key until it stops enlarging). If you can't use about:config right now, your changes were stored in a file named '''prefs.js''' (if your currently active profile folder). If you were to edit out the changes or hide/rename/remove that file, then Firefox could start up with normal sizing. Note that prefs.js stores other things like custom home pages and pinned shortcuts for the new tab page, so if the file is removed, those will reset back to their defaults as well.