... you're not - it's a known design flaw in web browser design: a global process running all tabs/windows in order to reflect a global javascript execution context. chrome fixed the design flaw by running entirely separate processes per window, which brings its own set of problems.
It's a also a design flaw in the HTML5 system itself: every webpage is basically its own program, so you end up running lots of program that are under the control of people whose interest is not to reduce your CPU or memory usage.
Most real applications try to be careful not to use up CPU resources at all as long as there's no user interaction, but many webpages have way too much commercial interest in constantly jumping up and down to grab your attention and/or staying in touch with a whole bunch of servers to maximize the amount of data they collect on it.
I almost regret the time when flash was the only game in town for "active" web pages, and I could just run a cron job to periodically kill all flash processes.
Stefan