Torbutton comes pre-installed with Tor Browser and we urge you not to change it.
We do not recommend to install it to Firefox because this is not a sufficient
way to surf anonymously.

Torbutton guarantees that DNS requests are sent through the Tor instance that
comes with Tor Browser. You should not change the proxy settings.

It’s strongly discouraged to install new Add-ons in Tor Browser, because they
can compromise both your privacy and your security. Plus, Tor Browser already
comes installed with two add-ons — HTTPS Everywhere and NoScript — which give
you a lot of added protection.

You can read more about it here:
https://www.torproject.org/projects/torbrowser/design/

Also have a look at this page for already answered questions:
https://trac.torproject.org/projects/tor/wiki/org/teams/CommunityTeam/Support_discuss

For other issues you should know about have a look at this blog post:
https://blog.torproject.org/toggle-or-not-toggle-end-torbutton

For a list of all torbutton announcements see
https://blog.torproject.org/category/tags/torbutton