I'm sure you've heard many warnings not to install add-ons or change the default TBB configuration, even from me. It's not that ZOMG if you install an add-on you will die. It's just that you have to understand the tools that you're using and their security risks, and most people are not competent to make that judgment, so the easy solution is to tell everyone not to fuck around with their settings. 95% of add-ons are probably safe to install, but nobody knows what all of the 5% dangerous ones are, so it's best for most people not to install any. These two have astor's seal of approval as being safe, if that's worth anything. Since Tails uses Tor Browser, yes. You'll have to enable the persistent volume to keep the installed add-ons.