If you changed custom User Agent and you want to revert it, the predefined value is
Mozilla/5.0 (BB10; Kbd) AppleWebKit/537.10+ (KHTML, like Gecko) Version/10.1.0.4633 Mobile Safari/537.10+
Install through F-Droid client to get auto-updates - https://f-droid.org/app/org.indywidualni.fblite