Doesn't work anymore
EDIT: As Javier said, use ycs-continued. That works perfectly

I used to love this add-on. It was pretty much fundamental to my workflow when watching YouTube videos. It's a shame that it's now abandoned and fails to work properly :/. If you're reading this Eugene, I really enjoyed the add-on while it lasted, and I hope that you (or someone else out there) are able to get it up and running again.

Edit: Just checked the Github repository and it appears that a fork was recently created called YCS-cont by pc035860. Glad to finally be able to use YCS again.
Edit 2: The author has now added the extension to the official Firefox add-ons page and it's called "YCS - YouTube Comment Search (Continued)" by Pymaster

Great extension. Unfortunately, it no longer works :(

worked for literal years and was super useful until like a month ago when it stopped working, it's useless rubbish now...

Extension has stopped working for weeks

Great extension, but it stopped working properly after youtube/browser updates.

it sadly stooped working

yeah it's true, stopped working

Two days ago, it stopped loading any comments and it can't search in anything.

Stopped working

Worked great for quite a while but changed rating to 1 star as it's been busted for a few weeks now. Clearing cache/cookies (as others have suggested) seems to fix it for a day or so, but then it stops working again. Will restore 5 star rating when it's working properly again. Having the same issue with this addon in chrome and firefox.

Stopped working. Please fix.

Doesn't work anymore

Loading comments does not work on Firefox, lack of updates.

Not working

On this extension page it claims it is Mozilla Public License 2.0, but on the linked Github site, it says it is MIT license. Further, that repo contains only the README, license, privacy policy and a few image files. There is no code there, and seems there never has been.

AMO add-ons do not have to be OSS of course, but I believe not providing the source is a license violation? The whole thing is shady.

This extension might be legit, but at the least the author should correct the license and provide the source code.

This could easily become a 5-star review!!!!!
But you have left out some super obvious functions:

1. I want to be able to sort my search results according to chosen column (Time stamp etc.). Now it just sorts on all comments.
2. I want to be able to reply in the results (alt. that you link me to the comment section so that I can post a reply to any filtered comment). Super annoying that I can´t engage in the comments. Might be over 1000 comments and I will not be able to navigate to where to answer.
3. It´s very unuseful that hits are listed in random order instead of chronological (especially transcript hits!). Let chronological order be default for any filtering.
4. Implement a feedback function so that I can send you these comments instead of writing them here! Could be as simple as leaving an e-mail or Twitter contact.

When these four points are done you will be awarded 5 stars!

Oh, btw, the 1st star is because this tool will find hidden (or censored?) comments :-)

Contains an XSS vulnerability, creator refuses to acknowledge that it exists, I'd avoid.

Stopped working now.
Please fix it!