I've created a distribution channel to get Snowl changes out to its testing community faster. I'll push development snapshots to the channel on a regular basis as significant fixes are committed (or perhaps nightly, if I can automate the process).
Once you install a development build, Firefox will automatically check for newer ones and prompt you to update when it finds one, just as it does for the release builds on AMO.
This is truly the slicing edge. If you use these builds, be prepared for the possibility of regressions and bustage. On the other hand, you'll also get to test new features earlier, like alta88's Places integration work.
Install the latest development build, then discuss it and report bugs you find. (If you don't want to be on the slicing edge, try the latest stabler release instead.)