Try about:memory, it's pretty good.
In Firefox I still see a lot of memory commited to tabs that I've closed long ago, and can't really get a breakdown of my extensions memory usage.
Also what I was wishing is something more like a os task manager. Where I could kill or restart extensions that are using too much memory like I can do to normal applications.
Simply stating that this extensions is using too much memory so don't use it (a la IE) won't help. If that extension is the main reason I'm using your browser I'm not going to turn it off.
I'm not saying it would be easy or even doable right now, it's just what I wish to see in a future web browser to keep the memory down and prevent me from killing it and restarting it twice a day while working.
Now, on the other hand, I have been wanting a real task manager in browsers for years. It was one of the first things that got me excited about Chrome.
Once that's in place, we can slap a friendlier UI on top, something more like the about:nosy add-on (http://www.visophyte.org/blog/2012/02/04/aboutnosy-is-aboutm...).
If you're seeing memory for tabs you closed a long time ago, we'd love to hear about it. https://developer.mozilla.org/en/Zombie_Compartments#Reactiv... has documentation on how to check for these cases.
As for measuring memory consumption of extensions, you're about the 1000th person to suggest it :/ New-style ("JetPack") extensions have some tracking, but old-style ("XUL overlay") extensions are really hard to measure, unfortunately.
If a critical extension is using too much memory, one option that might be suitable is to create a separate profile. E.g. Firebug leaks a lot (https://bugzilla.mozilla.org/show_bug.cgi?id=669730) and some people only need to run it occasionally. Depends on how often you use the extension, of course.