First of all it took me like 5+ minutes to get through your maze of distraction """""support""""" articles to get to login page to slowly retrieve password and username separately to get through maze of distraction articles again to actually report a bug to help make YOUR browser work properly.
Now let's get to actual problem. Copy paste isn't working in youtube comment section. That's Firefox' exclusive problem confirmed by other users. Changing "dom.event.clipboardevents" value on config page to false fixes this problem but it's also reported to break comment section on other websites.
Additional System Details
- Shockwave Flash 30.0 r0
- User Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:60.0) Gecko/20100101 Firefox/60.0
I had no issue to cut and paste on Youtube. I think you have more going on that is causing the issue there. And you need to give link where you mentioned "FireFox exclusive problem confirmed by others" for others to see what your talking bout as well.
I was having a problem with Paste for comments on YouTube; Copy and Cut worked fine. That problem started in late May or early June due to a YT update.
This bookmarklet works to override that YT fault.
Create a new Bookmark, name it something like YT Paste in Comments, and paste that code into the Location line for that bookmark. Click that bookmark before you want to paste into a comment text field.