The Tablet version seems to have greater support for addons, it's great to have Lastpass with me.
I've got an old Droid 2 and won't be replacing it until this fall. I try to run Firefox Beta, but it always takes on the order of ~20 seconds to start. It's quite frustrating! I much prefer the Firefox experience to the built-in browser with the exceptions of font rendering, poor hardware keyboard support [1], and the long start up time.
Edit: I just downloaded the update, and it's incredibly good. The startup time is almost instantaneous and the fonts finally look comparable to the Android browser. I'm quite impressed! I haven't tried using the hardware keyboard yet, but these two fixes alone represent a significant upgrade to the Firefox Beta experience. The new start page looks slick, too.
So I guess I've just replaced my default browser.
Edit 2: This is _really_ good. You have to give it a try, especially on older hardware.
[1]: Left/right keys don't work right, editing is buggy and places the cursor in improper places, modal keys get 'stuck', etc. I think it's a regression; at one point this wasn't a problem. I wish I had a bugzilla link, but I don't have the time to find/make a report.
I too have issues editing, I'm hoping that'll be resolved soon but even with those bugs it has been a better overall experience than the Chrome or the stock browser for me.
The new Firefox beta seemed to be quicker than older versions but crashed within a minute of browsing on a semi-heavy page, it is just too unstable and its memory footprint is way too large for a phone like the Nexus S.
The Firefox Android team put a huge amount of effort into redoing the UI to make things far faster. Support for Flash is fine for some (and much requested), but not the marquee feature in my mind.
(I work for Mozilla, but not on Firefox for Android)