I've been playing with Firefox Mobile Beta 5 on my N810 and here are my thoughts:

-I really like the convention of flicking to the left or right sides of the screen to bring up toolbars. Screen real estate is at a premium on mobile devices and this seems like a good way to deal with that fact. This sort of UI convention will be really important on the even smaller screened n900.
-I use Weave on 5 different laptops/desktops and was looking forward to this in beta 5, but it doesn't appear to be functional for me.
-Looking forward to mobile add-ons, especially ones to disable/enable slow flash objects on a selective basis and to block adds.
-page rendering is pretty slow and I get lots of random pauses when trying to scroll around pages. Page layout is not great on some more complex sites like Amazon. Definite room for improvement here.
-Lots and lots of missing settings dialogs. Having to go into about:config is not a pleasant way to set a homepage.
-I like the double tap to zoom in and out.

This is still a very early effort for Mozilla and I look forward to seeing a more polished version in the future. I can't see myself using it much over the default browser at this point.