I agree. Avoid going to 3.9 as Bud crafted his upgrade for a very specific firmware and hardware build. Updating firmware on older versions of the camera does install the firmware but not always in a way that mirrors the later models. There are recorded cases of FLIR upgrading forum members firmware after a repair and the camera firmware configuration being different to that of a camera that left the factory with the same firmware. Any differences could impact upon whether Bud's upgrade works of your camera crashes.
Yes, upgrading from 2.x to 3.9 and then applying the patches might bring you some trouble. I have a non-Wifi E4 that had FW 2.3 installed but new HW - apparently someone tried to downgrade the unit for whatever reason. Buds patches worked after applying the 3.9 FW, but I had to find and remove a redundant file to make the basic liberated menus work. Doing so requires some experience with FTP, file systems, comparison tools, fighting Windows nasty RNDIS ... it's not a newbies job.
If you're willing to get into some trouble, one may try Bud's patches on an old HW model updated to 3.9, but clearly not recommended.
Didn't try out the advanced liberation package yet, but many thanks to Bud for his great work.
BTW: Thanks to Fraser also, for pointing me into the E4 direction in some other thread ...