There's a couple things you can do to see what kind of overhead you've imposed on Firefox.
about:addons-memory Extension will give you a nice looking page showing how much memory each of your extensions is using. You can use that information to decide if the overhead imposed is worth the functionality it provides, or if you should look for a more lightweight extension that has similar functionality.
about:memory page will give you a very technical view of how Firefox is using the memory that's been allocated.
Be careful though, both views give fairly technical information and can be easily misleading if you don't truly understand what you're looking at, how memory allocation in Firefox works, and how your OS manages memory.
As far as I know Firefox doesn't have any built-in way to show the other details that Chrome's Task Manager does, like CPU and Network usage. As @RobK alluded to, Firefox's current architecture may not make providing such statistics easy (or even possible).