Some websites don't load at all (if they ever load), with this addon enabled, specifically wrestlinginc.com. It was probably also causing issues with Youtube, in the past it would load extremely slowly (over 1 minute plus hang my 32GB memory pc), although lately it would only hang for 15 or so seconds. I didn't think it could be caused by this addon, till now. I just tried youtube with it disabled, and it loads very quickly with no hanging.
I think something with the way the addon is injecting css/js into the page for controlling zoom on the website not working correctly with whatever the css/js the website is using.
My Frefox memory with only the above website homepage loaded with the addon disabled is under 600MB. With the addon enabled, I had to kill the process after it using eating over 2GB of memory, and still hadn't loaded the homepage. Its the same problem with other pages on the website.