I don't know this for sure, but a friend of mine had installed FF and used it for several hours, then he left to run an errand, and when he returned his system had froze. When he restarted, some of the settings he had made were intact and some were not. Now I know this is a pretty basic story, but in my own personal experience, even if FF is active during a freeze, my settings have always stayed intact. Weird!!
Concerning extensions, there are some do's and do not's just as with everything. If you install only the higher ranked extensions, more than likely these will be compatible with the latest version of FF. Also, read the user reviews, and be objective. If you read a review similar to yours that had trouble, pass on that extension.
I currently have 58 extensions installed and activated. I test quite a few so that number fluctuates, but I keep tabs on the ones that I use no less than every other day. Several of them enhance features of other extensions.
The nice thing, is that even though I have alot of ext., my toolbar is still only an inch or so tall, as a big toolbar annoys me. I guess results vary PC to PC, but in the time I have been using FF ext., I have only had to reinstall FF once, and thanks to a couple of extensions that I use, reclaiming my settings, bookmarks, extensions, themes, chrome, about:config settings, and all the other info I like, was painless.