Firefox 3.7 will become 4.0 -- Firefox 4, with updated Gecko, confirmed for later this year
It's a far cry from Chrome's rampant and reckless versioning, but I wouldn't be surprised if the 3.7-to-4.0 switch is perhaps a sign of Mozilla being reactionary.
Still, let's take a look at what we know about Firefox 4.0 (or 3.7) so far! 4.0 isn't much different from 3.6.4. There's Aero support and a new theme (Strata!), so visually it will look different -- but under the hood, 4.0 is basically a vehicle to update Gecko to 1.9.3. Of course, with a new version of Gecko we're going to get faster and more standards-compliant browsing -- but I wouldn't expect 4.0 to revolutionize your Web experience. 4.0 will bring us Jetpack though and a whole slew of new, faster, no-need-to-restart add-ons. By the time 4.1 or 4.2 comes around, Firefox could be back in the hot seat.
If you're interested in what Firefox 4.0 heralds, give the 3.7 alpha 5 nightly build a test run!