At least they're not binning 3.6.x yet
While our main family machine (an 'experienced' Pentium D 3.4GHz) is happily running Firefox 4.01 on Lucid Puppy 5.25, the same machine running XP SP3 is still stuck on Firefox 3.6.17 as ZoneAlarm apparently isn't compatible with Firefox 4 at the moment.
Meanwhile, the main browser I use on my reconditioned AMD K6 2 500 is Opera 11 on Lucid Puppy 5.11 - it's a considerable improvement over IE6 on Win98 which is the other main OS on that machine.