I am wondering if I'm lucky but my foxes are always fast, even with 100 tabs. Both on Linux and Windows boxes.
You are not the first one who complained about the speed here, so I am wondering whether our difference is due to bad luck or good luck.
But there are people that experience the opposite and it's usually due to a shitty extension they have installed. Firefox will fall back to software and disable HW acceleration if an extension is coded like dogwood. In that case the user should check in "about:support" to make sure their GPU HW composting is enabled, that's usually the culprit. "about:performance" will list the extension that's causing the performance issue. Another issue is that Firefox has a hard time rendering Facebook in comparison to other browsers, since most people have Facebook loaded at all times it would for sure give them the illusion that Firefox is slow. I am still looking for a solution for that issue...