If your a power tab user, then Firefox is the only browser that can keep up with the 80 or so tabs I always have running, other browsers will will halt to a crawl if I attempted that, either immediately or within a couple days. Firefox allows me to power tab my way through a couple months before I have to restart it, it's just that good.

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...