The previous beta release of Firefox 4 for Android was horrible. It used to consume too much memory with only when few tabs were opened and the browser was laggy.
The latest beta release, it much better but still not perfect for me to use it as an everyday browser. I still prefer Dolphin Mini over this heavy browser.
I have played around with the browser for some time now, opened multiple tabs, and it definitely takes lesser ram comparatively to the previous beta versions. Scrolling is smooth, but there are regular hip cups every now and then. Here is what the official logs say:
Continued improvements and fixes focusing on stability and usability
Firefox will now move its profile data to the SD card when you move the application to the SD card on Android 2.2 or higher.
Firefox for Android comes with a sync function. The sync function allows you to sync the following between your Android phone and desktop Firefox browser:
Syncing for the first 4 options mentioned above has practical use. It makes it more convenient to take your desktop browsing experience and carry it away on your mobile device. You don't have to bookmark everything again, neither input your login credentials again. I personally don't find any practical use of syncing tabs between the phone and desktop browser. With the sync tab option ticked, you will be able to see the opened tabs on the desktop from your phone and vice versa. For using the sync function, it is assumed your default browse is Firefox on the PC, otherwise it does not make any sense at all.
I would still recommend Dolphin HD or Mini browser over this, but those you are hardcore Firefox fans, can give this a try. Hopefully the final released will deliver a much smoother and better experience.