I agree, Youtube was always slow to load, partially because it 'fills in' so many elements: the thumbnails of other videos, comments, the playback controls, etc. But with Firefox (until last week or so) they all kind of filled in pretty quickly, and I had many more tabs than the minimal 5 I have now. I had my main Youtube channels, plus some extra channels and random videos I had saved in that particular session. To make things easier, I slimmed that down to the most commonly used channels (and the lists of videos, not actual videos, so it doesn't try to play a bunch of videos at once, which mucks up any browser on any machine I've used).
SaveTube sounds awesome, because all my previous video downloaders have lost functionality, so I'm eager to try that one. In reading the site, I see that I supposedly can add that from the Mozilla Add-ons link, but my experience has been that the version of Firefox I was using on the Mac stopped letting me install add-ons. I think it has to do with certificates, etc. I encountered that when trying to install various adblocker options mentioned in another thread here. In looking at the installation and usage instructions, I think the script stuff is far beyond my ability to execute. Someone had previously recommended Youtube-dl to me, but I couldn't figure out how to operate it.
From reading the ViewTube stuff, I'm not quite sure what it does. I don't want to embed Youtube videos or watch embedded YouTube videos. Is it possible it allows me to use something like VLC to stream Youtube videos? That might be a workaround, but one that would be somewhat time-consuming to load the video on SpiderWeb, activate ViewTube and then play it via VLC. Somewhat clunky.
I see that the user agent switcher is in that add-ons folder? I'm going to check.