I don't have a way to test, but if the developer of the add-on doesn't provide support, you could consider other, similar extensions. This one showed up in a search:

https://addons.mozilla.org/firefox/addon/ultrawideo/

That one was updated in October. I haven't tried it myself.

Edit: Here's another one:

https://addons.mozilla.org/firefox/addon/ultrawidify/

That one was updated in September. I haven't tried it myself.

Hello thanks for response,

About Ultrawidify - i alredy tried and is do not streach the image automaticaly on youtube, so you have to click evrey time on video settings/ streaching/ basic, but when u run another videoclip you have to click again to work.

About ultravideo i tried right now it work automaticaly on youtube, ... but is cropping image, is do not strech the image..,cropping is not so good actually is zoom magnified all the image sometime is cutting from the head of people u dont see all the image. That way i prefer streached image cause u see all the image on 21:9 even is streached a little bit

About Ultrawidey is streaching automaticaly image on youtube. Also if you press Z key then is crooping image, if u press Z again then is streaching image again, so u have bouth option, and it also working automaticaly I dongrade to firefox last version 63.03 cause there is working automaticaly. After update to firefox 64 automaticaly dont work anymore only if u press X key evrey time on evrey videoclip

Here's the thing you didn't notice:

Last updated a year ago (Dec 12, 2017)

This might be a big reason as well.

Cool news!!! The developer just updating his aplications Ultrawidey on 23 december 2018 and right now is working again on firefox 64, thanks all for responses.