I went ahead and downloaded the Firefox2/fennec and i've been noticing I keep getting an update notification for the same version. Something like ver.40b3...or along those lines. I can't remember exactly what it is. I usually do go ahead and update anyway, but I was curious to see if you were getting the same thing. It has definitely come along way since the first release, and i'm looking forward to using it as my go-to browser.
The last release of Firefox for Android is version 4.0b2. The next release will be 4.0b3.
"4.0b3pre" is the version of the current nightly builds. These are updated every night with the latest development code. They are often unstable or broken, because they are snapshots of our work in progress. If you use the nightly builds, then you'll receive a new update every day.
If you'd rather have a more stable, tested experience, you should install the latest beta version instead. We release betas every few weeks or months. If you install the latest beta, then you'll receive an update whenever a new beta or final release is available.