There is a long discussion (covering two years) as to the pros and cons of including uBlock Origin as a pre-bundled part of the TBB (https://trac.torproject.org/projects/tor/ticket/17569), but no conclusion was reached. In my experience with normal Firefox, adding uBlock Origin gives a huge improvement in browser performance when going to heavy websites (think CNN.com for example). This is more critical with TB due to the slowness of the Tor network. So I would like to add uBlock Origin to my installation, but of course I am concerned that there will be privacy implications.
Adding addons is not advised for several reasons. Why not using what already exist in the TBB: NoScript and Security Settings. You can block many scripts from running with those tools.
This apparently comes down to browser fingerprinting. How it's possible to track one uniquely via browser extensions, I don't know the details.