If they cared and their slowdowns were simply related to addons, they could always sample the overhead of said addons and show a warning. My moderate use of addons in FF didn't feel too aggressive, but then again I'm not in the clique with dozens of open windows. There were times of fast/slowness in FF. It was never bad enough to chase me off the platform, but to say the slow ess was simply saddled on addons is probably disingenuous.