My favorite YouTube Add-on (for both Firefox and Chrome). I've had poor experiences in the past with other developers' extensions (Looking at you, Magic Actions for YouTube). Many of them have very questionable "features" in their software, which spy on you, create popups, etc. This software is free (as in freedom) and the source code is available on GitHub! (The very paranoid could use that code instead of the binary, for 100% knowledge of the code s/he was running.)

Much thanks to the developer for these excellent extensions!