Firefox just added addon support officially on mobile. All other firefox browsers that enabled them in the past do not differ in this regard anymore.

Focus and klar are the same with a diffrrent name: wtf firefox? It's a very lightweight, very fast browser which enhances privacy a little bit, especially locally.

Nightly is the developer version of firefox if you want to help firefox and discover bugs use it, you won't experience many bugs though. Unless you really use the latest features. There may be other bugs though, use at your own risk.

For the serious privacy minded person you should use mull or at least fennec. If mull is too hardened for you, you can soften it again by disabling the settings.