Hey Michio, I assume you're the one asking the same question on MacUpdate? glad you made it over to the forum. :)
Imo, you have to differentiate with what set of data you do what. The fact that you need a gmail account to access extensions in the first place is crazy and tells us a lot about how Google works and what they think. They most likely think that humans are some kind of glibberish mass which provides data and Google will happily collect all the data possible to one day take over earth. :P But seriously, you can still create a fake gmail account and log in whenever you need an extension. Then you can log out again, as soon as the extension is installed.
Keep in mind most extensions will have access to your browser history, sometimes even more than that. So you shouldn't just randomly install extensions. I only have very few but I am aware of the fact that this is of course in a way compromising my privacy. We don't wanna talk about Google Chrome - that is much worse!
But the same goes for Firefox if you e.g. use AdBlock or similar add-ons. Only solution: don't use browser extensions at all. GlimmerBlocker (works through a proxy, I think) might be a workaround. I'm myself am too stupid for GlimmerBlocker. I wish they had a user interface which was easier to use.
Hope this helps,
w.
(account deleted)