Palemoon is described as a version of Firefox 'optimized' for Windows OS's, meaning some stuff has been stripped out, I would assume, and other stuff 'tuned' for Windows.
Even though PM4 is recommended for Win7 & PM3 for XP, I've been running PM4 on 2 XP rigs for quite awhile now (currently 4.0.7) without any issues and I prefer the PM4 UI. I had used FF for a long time, but FF4 and FF3 (starting with FF3.6) were both causing me headaches with crashes on opening multiple tabs on one of the machines which is what led me to try PM. Why PM has no trouble here & FF does is a mystery, but clearly some of the stuff 'stripped out' of FF to make PM may have been contributing.
PM works with almost all the FF add-ons. A number of sites I need to use regularly (mainly Outlook Webmail & some banking sites) don't render properly (or work at all) on FF or PM, but using the IE Tab Plus add-on fixes that very nicely.
I've tried Opera, Chrome, Safari, IE8 and FF3/4, and PM4 is where I settled. Works great & is very stable, even with 10 or more tabs open.