edit: please keep your promise
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hi, you 1. click the 'Improve Youtube!' (extension icon, top right of browser),
2. 'Player' 3. 'Rotate' (you will find a new button in the youtube player)
-2. Alternatively you can just Type 'Rot to enable the feature
( please understand we are hardworking volunteers and blackmailing with one star rating is not right. There should be big reason. It can frustrate us for a long time. Even with 4 stars the pressure is real.)
many smiles

:( hi! sure? that must be a rare bug then. Please join us at https://github.com/code4charity/YouTube-Extension/ . The project deserves 5 stars. We are working hard (and for free).

2022: Sorry to notify you late, but please try again!! :) (We found a great solution and there is no alternative)

2021: Hi, this feature was fixed / completed in our new version at https://github.com/code4charity/YouTube-Extension. Our Firefox version started recently. Yet we look forward to update the Firefox version soon and often.
I understand 'reverse playlist' is a legendary feature as people miss it. (Because other projects, providing this feature only, resigned, despite they had many users. Because youtube changed this repeatedly.) Thus we temporarily removed the feature from our descriptions.
Yet that is not what you can possibly rate here. What you are rating can only be our whole work with 100 features and an insane amount of work. That's how we always called and describe. Yet your rating actually has a (negative) impact.