This is a problem with standard Firefox as well where background
tabs and windows can continue to have active content, using CPU,
even though you aren't actively interacting with them. There is an
add-on you can try (
), but it is highly experimental, and is known to have some
problems on certain sites.
The long and short of it is that Mozilla isn't sure how to solve
this problem, and I'd rather not have us work on a half-cocked
solution of our own because of the compatibility risks, though I
certainly acknowledge the issue.
I'm testing the "Suspend background tabs" add-on right now.
Largely it seems to work as expected. For underpowered Macs it's
probably a good solution. It can be disabled without re-starting
the browser, in case it's interfering in some way (which it most
certainly will at some point). BTW, the guy who makes it is also
responsible for AdBlock Plus, so I'm confident the add-on will
improve over time.