[IanChiles]

Firefox as of 23+ uses significantly less memory than Chrome (in my experience - your mileage may vary though)

[kolev]

With same tabs open for the same amount of time, Firefox uses significantly less. I hate to say this, but Safari beats Firefox, which makes it the most efficient browser on OS X.

[skeletonjelly]

Being able to identify which tabs are hogging CPU/memory is a deal breaker to me.

[Grue3]

about:memory shows detailed report on which website uses how much memory.

[scholia]

I suspect that Microsoft Office uses less memory (and fewer processes) than Chrome ;-)