I can foresee this becoming my savior for the video size, which is just overwhelmingly large on desktop. Sadly, the video progress bar seems to not match the width of the video, so a 4 minute video will only show a clickable progress bar up to 2 1/2 minutes with the rest of it hidden. I assume the progress bar is still the length of the new video width with a big portion of it just cut off.

I actually found yet another workaround for Good Old YouTube and Custom UserAgent String (*edit* now dead as of this afternoon), but the Add to Playlist button is broken on video pages for that. Still need it desperately for playlists (which are also hideously oversized and slow-loading compared to the fast-loading, compact lists), which literally don't load without constant Firefox warning messages. Huge problem if you have thousands of videos in playlists that are now unloadable. I've been making constant Save Page As backups while it lasts.