Hm, I read a lot of critical words in this thread. Firefox for maemo5 is not in a final state but it's significant improved. My impressions:

+ scrolling is nearly as smooth as it is in microb if the a site is rendered completely
+ featured extensions are working fine.
+ AdblockPlus works without complaint and is configurable in the same way as it does in the desktop version
+ youtube works fine

- Initial start takes more than 10 seconds - that's the most annoying thing for me
- If AdblockPlus is activated it takes some extra seconds
- scrolling stumbles if site is not rendered completely
- On screen keyboard does not work correctly - this was better in older releases, as Milhouse already wrote