hi guys! Iridium is NOT UPDATED 3+ years, so a lot wont work.. Try Improvedtube.com though. ( ​https://github.com/code4charity/YouTube-Extension/blob/master/README.md ).
, it is the only big Youtube extension developed openly and since several years and Recommended By Firefox🦊 (the alternatives resigned along the way, because it requires continuous work)
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Worked great once, but a year ago the dev got mad when asked to design the extension with the same permission standards as other Firefox extensions, and then just stopped working on it entirely. If you must use this, use an older version, but its not maintained any longer and just causes more issues than it solves.

I had trouble with it before, it was just a setting problem. Now it just completely stop me from watching YouTube. Spent a good 10 mins disabling my extensionsto find out which one was the problem. To my surprise, the add-on meant to improve my YouTube experience caused me the most problems on YouTube. I don't know what they did today, but they broke it.

Save yourself the headache and don't use Iridium.

It was great when it came out, and finally it was a working add-on that could reverse your playlist, and then it suddenly died.
Sadly, the author has been occupied that he or she couldn't update/fix this anymore, so... consider this abandoned.

add-on went to shid a year ago when they decided to totally redo the UI and then just stopped updating it

The writer freaked out because he had to follow the rules and then he abandoned this extension. This is mostly malware now.

Needs a LOT of work before I'll ever consider it again. Intrusive constant pop-up asking for money...NO FUCKING WAY. That's why extensions like this exist...we don't want ANNOYING BULLSHIT like that!

Great extension, an absolute must-have, unfortunately Mozilla has recently decided to be extremely anal about it, so it's no longer supported here. Go to the developers github for newer versions

21.4.2020/// As of today, Iridium completely breaks YouTube's autoplay. Keeps on giving a "playback error". Forced to disable it for now.

Used to be a 5/5 app until the recent updates which removed all of its functions.
Go back to version 0.2.5 if you want this addon to work properly.

IMPORTANT: Use old version!
This is a great extension with many useful features. There aren't any alternatives offering the same quality. Amazing work!
HOWEVER, due to some issues explained on the GitHub page, most of the functions are gone in the current version. Until the functionality is restored, you should use the latest "old" version for now, which is 0.2.5. You can find the version archive under the "More information" section of the store page.

Used to work fine... right now though, it just completly breaks YouTube for me. Not a single YouTube video wants to load while this add-on is active.

It does everything it advertises and blends in so well with youtubes UI. I can't wait till they add more features.