I ask because that would be a bad info leak and could be used to identify someone uniquely instead of just a Tor user in a sea of Tor users.
On that note, can I spoof them or hide them if the answer is yes?
You can spoof them, yes. And - there's no problem in detecting add-ons in the Firefox bundled in TBB. It's a regular case, and if you're using TBB instance of FF it's more than detectable, actually. It's because of the fact, that it LTS-like version, not an ordinary FF. It states lower version, but due to the fact, that many patches were backported to it in it's development cycle, it does not have some bugs usual for an old version it states. So - it's pretty clear that you're using TBB FF, elaborating some JS tests, for example.
It might be a good idea to just use the plain Tor Browser and not install future Add-ons.
How visible an add-on is greatly depends on the add-on itself. If it blocks ads then one will be able to see that you don't load ads, if it "phones home" then the IP might be the one from Tor (or not), but it still might create some kind of ID that is unique to you.
I'd suggest not to use any Add-ons other than the ones installed on the Tor Browser per default. I don't think it really is possible to completely prevent any potential information leak of any Add-on out there.