I use Firefox with a lot of extensions (25 at the moment), and I've never seen that on Youtube, not even when I have 100+ tabs open. Have you tried the 64-bit edition? In my experience, it's far better than the 32-bit, and has been since the first beta release of the 64-bit version for Windows.thats the only reason I took so long to switch to begin with. I don't want to use chrome, but I need to be able to open a youtube video without my browser hanging for 15 seconds.
It's frustrating indeed to see Firefox trying to be Chrome, but to me, it's still the only choice. I wish I didn't have to use addons like Classic theme Restorer to fix Moz's many boneheaded design decisions in the last few years, but at least once I've done that, I can still get the UI and features I require. I used to use Pale Moon (before FF was available in a beta 64-bit edition for Windows), but ever since it started to identify itself as Pale Moon rather than FF to addons, many of the ones I use haven't worked, and FF x64 (beta or otherwise) has soundly beaten every x64 Firefox derivative (Waterfox, Cyberfox, Pale Moon) in performance (as measured by Peacemaker).