Besides the faster JS engine, faster browsing, better html5 support, etc, one of the major things I was looking forward to in firefox4 was the tabs on top interface which is the standard in every other modern browser except for safari (I think). I just think it looks nicer honestly, but the tabs also get some gradient transparency in this minefield build. The main disadvantage to running minefield (as opposed to 4b6) is that most extensions aren’t compatible. If you’re only wanting to run the betas (minefield is for all intents and purposes, pre-alpha), you’ll have to wait for early october for the new linux interface. Still, looking good so far.
The Blog of Lanteran 