Firefox addons are known to send data (like the URLs visited, location, installed cookies etc.) to their maintainers and (from there) to third parties. See for instance the newest "scandal" about the WebOfTrust addon (link is in German):
Since, on the other hand, addons are also essential to prevent tracking activities (e.g. the well-known NoScript addon), simply disabling all addons does not seem to be a good solution to protect one's privacy.
What I'd like to do is: Keep my privacy protecting addons, but somehow (maybe through Firefox internal settings in the about:config dialog or some hack to Firefox or indeed to the operating system, Debian 8 in my case) prevent all Firefox addons from sending data from my computer to the internet. Obviously, this should not be achieved with yet another addon, then we've come round in a circle. (As a downside I realize that this will also break auto update of these addons, and maybe even their normal functionality. But in the latter case this would be a reason for me not to trust them any more.) In fact, I'm using Iceweasle on Debian 8, but this is synonym with Firefox, I think.
(An alternative would be to review the source code of the addons myself line by line and then - after convincing myself that they do nothing more than I expect them to do - compile them myself. But since I am not an expert in this field (I don't even know how to compile a browser addon) and since some addons try to obscure their activities deep inside the code, see for example:
the blocking of outgoing traffic initiated by addons is probably more practical in my case.)