Good

Great addon, a must-have if you're using YouTube on a regular basis.
Question: is there any chance for a mobile release?

ok,ok,u said you're swamp with work,and can't do this for CHROME,now you're at home CORONIA VIRUS IS EVERYWHERE,SURELY,SURELY WE CAN FIND THE TIME,IT NOT LIKE YOU'RE GOING ANYWHERE AREN'T WE??

***UPDATE 3-31-2020***
Switching to new theme fixed it! Thank you. I don't like the new theme but as we all know they will be switching to it eventually so I'll just deal with it.

This add-on was working fine for me and then it was updated (yesterday?) and no longer does anything. I've tried uninstalling and reinstalling and syncing the cache and it shows the videos I've watched on the settings page but it no longer marks them when on YouTube when trying to select the next video that I want to view. I wish there was a way to roll back to the prior version.

*** UPDATE - 03\30\20 ***
Simon,
You are truly a scholar and a gentleman. First time any developer on the Firefox Add-On board actually responded to my review; but I am glad you are "alive" and came out of slumber to patch this. It is now beautiful and YouTube has a bad habit of recommending videos I have already watched MONTHS ago. Your GRAY overlay with WATCHED text is AWESOME; because I code while I listen \ watch YouTube, so this makes it easier for me to avoid realizing I am re-watching something TWO minutes in. We MUST stop these YouTube creators from dipping 2, 3, 4, 5, 10, 100X for the same videos, hahaha... Thank you and now all we need is a way to auto-sync history across all our machines with a single account. I am too lazy for Manual Export\Import.... FIVE STARS !!!

*** OLD REVIEW - 03\24\20 ***
Developer needs to update this add-on; but it is possible they have not tested it recently with Firefox, since it has not been updated for over a year. It is an AWESOME idea; but does not work correctly with Firefox 74.0. All thumbnails appear with a grey overlay above it with the text WATCHED in some cases. It is an obvious alignment \ overlay issue; so most likely YouTube "broke" it and this needs to be fixed ASAP. If this is fixed, then I will upgrade to FIVE stars; because it is needed, so now I am off to find another add-on.

It breaks the thumbnails on the home page, otherwise it works fine.

good because it allows exporting everything and importing it later, and it is actually base64 encoding so we can edit it with text editor maybe, hope extension for chrome and opera, bad because actually shows wrong videos in sidebar on right being watched, misalignment, but luckily this sidebar can be hidden via another extension called Youtube Enhancer

I just started using this. I also have the bug with the double-sized thumbnails but since I never use the Youtube starting page and the issue is not present in a channel's Videos page, it does not bother me too much.
I honor the idea of this add-on, as this is basically a must have.

Two questions or feature requests, though:

1. I would really like to be able to manually mark videos as "watched" and be able to de-mark them. Not sure how this could be implemented, GUI-wise, but I think it would be a very great feature!

2. Currently, it's sufficient to click a video to get it marked as "watched". Wouldn't you consider it more intuitive if a video only gets marked when watched "completely" or at least to 2/3, 3/4, 9/10? I know that's a bit philosophical and probably tricky to implement. Just asking. :)

This addon is godsend. However the latest YouTube update broke it. On hover / scroll the video thumbnails double in height every now and then.

Just a heads up, Looks like the most recent update to Youtube vaguely breaks this add-on. All the video thumbnails on the homepage sorta... double in height for some reason, at least that's been my experience.

Please, never leave me!