Did that and weirdly nothing seems to be excessive... indeed the Macs own performance monitor doesn't suggest anything is particular excessively using cpu or ram but here it is juddering away especially when scrolling pages.
Three year old Mac btw... everything else runs pretty well... if I get a chance I might fire up Firefox in Parallels and see if it's a Mac issue
When you write "about:performance" to your address bar, and press enter, you should access to the internal performance monitoring page of Firefox. That should list every tab and extension by RAM use and power impact.
Setting aside the fact that it's as fast as or faster than Chrome, it doesn't crawl any of my machines with >500 tabs (this has 562 as of now).
If you want to dig into your performance numbers there's "about:performance" to see what is using your processor and RAM.