I wonder if it would still happen if you added a "-" after the t in the youtube link. So if it was youtube.com/somevideo you would change it to yout-ube.com/somevideo. This will remove ads and comments I wonder if it would fix your issue.
14 | a day ago
Yes everyone :(
Recent thread "YouTube No Translation (addons.mozilla.org)" https://news.ycombinator.com/item?id=44613491 (219 points | 14 days ago | 103 comments)
And perhaps one or two more threads.
PS: You can go to the top right, click on the three vertical dots, and select "Language: English US". It has the side effect of translating the videos in German to an annoying and artificial voice in English, but you can switch it back.