I really don't see any improvement over FF4 on Android. It's still as bad as ever.
I hate the interface and it's still the same. Swiping right brings up the open tabs and swiping left brings up the some navigation controls and tools. I hate this design because I'm always opening these side menus when I don't want to.
Dolphin has a similar left/right swipe action, but in Dolphin you have the option to turn this off, helpful if you are dealing with a page that needs a lot of horizontal swiping (like google maps). But in Firefox you can't turn this behavior off.
I really love Opera's interface. No annoying side menus, everything is at the top in a compact bar.
But Firefox's worst problem is it's bloated size and slowness. It's dreadful waiting for the browser, waiting for everything. With Dolphin or Opera, I can get to the page, read an article, and wash the dog in the time it would take Firefox to launch, and load the page. Okay, it feels like it sometimes anyway.
Firefox is my favorite desktop browser. And I LOVE having my desktop bookmarks easily accessible on my phone and tablet. Also being able to see my open tabs on the desktop or laptop from my other devices is real nice too. So it's really disappointing that everything else about the browser just sucks on Android. I just can't use it in it's current condition.
Mozilla, you need to burn this, it's not salvageable. You need to start over from ground up and redo everything.