No work:
Transcript video and comments.

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

Not working anymore, but search the addon ycs-continued is a continuation of the original YCS and 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

dude where are you @Eugene?
this was such a great addon, why abandon it?
people have given up on you coming back, and started looking for ways to fix it themselves only to find out that not only have you NOT made the code available; it's also unclear under which license it is working... is it mozilla public license or MIT, is it opensource or not... i hop those saying you died are wrong, i hope you're in good health... all i would want to ask from you at this point is to please give us a sign of life and maybe an answer as to what is going on with this whole situation plz

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.

Used to work great but has recently stopped working altogether. Doesn't load comments anymore, just video transcripts

One of my favorite Add-ons. Allows me to search for specific comments quickly.

Stopped working. Please fix.

Doesn't work anymore

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

Love this add=on! Note that the Instructions Step 2 says " Find the YCS extension under the current video" The way to find it is to search for YouTube Comment Search or scan the page for the Load All button.

Not working

One of my favourite add-ons, but unfortunately it has stopped working for me a couple of days ago. Only the Transcript is still loading. Will give 5 stars again if someone can help me fix it.