There's no 'maybe'...
... this is definitely an off-store app so checking the option to install from anything other than Google Play is needed. It'll need to be left that way or rechecked with each alpha update, although fans of the mobile beta path who don't fancy opening up the install sources are covered as that is in-store.
It's different, but is quick. I run the FF beta on desktop and have been using this alpha for a short while on mobile. There are a few glitches (it sometimes stalls when entering text on screen - try editing a search phrase on the FF store) but overall it's steady on this SGS3...
Given that many apps just open a web view anyway, I don't see much difference in the way the FF ones operate. Hopefully by using a full HTML5 client, slightly clumsy stuff like the My T-Mobile one won't get stuck when spawning a page (they'd need to rewrite it for this platform, which is another issue all together).