Something that I find useful on Firefox is troubleshoot mode. It's under my 'help' menu. If that doesn't work there's probably something going on with the web site.
Youtube has been making a lot of changes recently so I wouldn't necessarily blame your browser. The Youtube Roku app recently would start playing the video in the thumbnail preview if you moved your "cursor" over that video. Really annoying, but I found a setting to turn it off.
I have also noticed that on my devices, and it's a positive from my POV.
There's a little speaker icon there; besides the closed captioning, if I find I want to "play" the video, I do the hover, click the speaker to hear the dialogue and there's NO COMMERCIALS!
Young enough to remember doing it;
Old enough to wish I could do it again.
I wasn't aware of the no ads thing with the previews. I pay for Youtube Premium so I never see any ads unless I am not logged into Youtube.