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.

No feature of the add on work, it's complete useless

I loved this extension because of the wealth of features. Until the features return I have uninstalled and gone with a competing extension.

for pity till blacklist features comes back, like in Youtube Plus, this extension is no better than any of several others.

EDIT 2020-02-20: LOL, Even with new YT layout UserScript "YouTube +" concurrently with UserScript "Youtube Hackz Magic Fixer...("Classic Youtube Layout")" works.
I do not understand why Iridum was created and why use it ...

old comment:
Does not work... Video can't play after install Iridium...
The most funny and the biggest irony is, that userscript the "YouTube +" (see github.com/ParticleCore/Particle) works well... (I'm using Tampermonkey.) Are You sure that the Iridium is former the YouTube +? It does not look like that...

Just waiting for "Reverse that stupid playlist" button. Thanks for Dark Theme and Disable Autoplay features.

Good extension that I have been using for years on Chrome.
However, the "Allow loudness normalization" feature does not work on Firefox. It is missing in the latest versions, and it does not work on version 0.2.5.
Really hope that this can be fixed.

Completely useless.

Thank you Particle for being amazing. I've been using Iridium for so long. I hope that my favorite feature, reversing playlists, comes back someday soon.

I don't have a stable income yet, but once I do I'll support Iridium. Thank you so very much for the amazing Add-on.

Unusable in its current state. The dev is still working on it on GitHub tho, so it may come back.

I mostly just wanted this to blacklist channels, cuz blocking them doesn't seem to work.

Disable hight frame rate feature doen't work in both old and new version.

Works flawless. Although not all features from the older versions are implemented, those which are implemented work flawless, as expected from the author who does an amazing job. I'm looking forward to future development.
Too bad most people give the rewritten version of this addon a negative feedback, but the fact that he now uses a Firefox API to more reliably inject changes to YouTube video player is a very good development. This strengthens this addon and will lead to a much better user experience in the end. Remember when similar addons which claim to disable autoplay actually only pause the video after the page was loaded and if you were unlucky the video was played for first couple of second anyway? (until page loading was finished) This will not happen with this improved injection method. It is really great. I enjoy this so much to never have to worry about any videos starting without my permission. Furthermore this will strengthen Firefox as a browser because this new API is only available in Firefox, not in Chrome.
I'm greatly impressed that the author had the courage to overthrow all legacy code and start a new version of his addon with improved techniques. This is something a lot of developers are afraid to do, and therefore their software dies in complexity.
I cannot understand why people complain because the old addon was broken anyway by recent YouTube changes. So there was nothing to loose.