I overrode the About:Config setting, Extensions.CheckCompatibility setting (after first creating that setting), which allowed PWM to be installed in Beta1; I had given up on the RC3 release. I suspect that I then had to use MR Tech Toolkit to make it compatible, as most other extensions I've been able to install required both steps.
No loss of master password, I'm happy to report, because when I first started using PWM I entered a very long master password and, so far, the programmers of PWM have not made it easy to change the master password; thus, were I to lose my master, I'd have to use up quite a few of the keystrokes stored in my keyboard and have to go to the store to buy some new keystrokes....
(I logged on here with no problem, thanks to PWM, as I'm using Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9b1) Gecko/2007110904 Firefox/3.0b1 - Build ID: 2007110904, which is the first actual Beta Release.