There are three categories of addons
Addons not needed
The tor-browser-bundles contains some additional fixes to fingerprinting techniques. You can read more when you lookup the "resistFingerprinting" Firefox setting, which uplifts quite a few of these settings.
Example: You do not need to change your user agent string, as the tor browser automatically uses a common Firefox user agent.
Addons still needed
Example: uBlock Origin can still block a lot of things, which may improve your privacy and your comfort reading websites.
Again take an user agent changing addon as example. The tor browser uses a common user agent, which is the same for all users. The addon may try to use a common user agent, but it differs from the tor browser user agent string.
Especially when you visit hidden services, which see almost only tor browser user agent strings, the addon will make it easier to track you.
And keep in mind, that there are many addons out there, which may be insecure or include their own tracking mechanisms. The less code you run, the better.