My observations, using both Firefox (well, Iceweasel 20.0-1~bpo60+1), and Chrome (Chromium 26.0.1410.43-1). In many ways, I prefer Firefox as a browser (better plugins, controls, tabs, too much Google influence already). My Firefox experience is getting to the point it's virtually unusable, with very, very slow response to any actions (especially new tab / new window functions). We're talking about many seconds, some…
Responsiveness being : it takes too long to a) switch tabs b) open a new tab c) access a menu (e.g. File/Whatever). The UI might take a second or more to respond. I might have 50 tabs open spread over 5-6 windows.
Currently, the browser is running well and I suspect the most recent responsiveness trouble was caused by an add-on (Brief - feed reader) which I have disabled.
But I second the call for a better way to identify badly behaving tabs (CPU as well as RAM) more easily.
Firefox is a great browser and thanks to all who make it.