Firefox is great, when it's not consuming memory and taking 90% cpu time.
I've found that it usually doesn't take more than Internet Explorer. Unfortunately, you don't have a choice not to duplicate system resources; you're running IE whether you want to or not. If you're finding your CPU is being hogged, the problem is with the source, not the messenger. ESPN.com's automatic streaming media content, for instance kills my old laptop. Try adjusting your settings to have control over what is running.
Another issue I've seen is certain viruses get embedded so that they are reported like a function of your browser.
Did a little research: IE may be better at sharing resources, or "backing down" at competing for microprocessor time. If you are sure your computer is virus free, try this:
Hit Ctrl-Alt-Delete to open your task manager.
Click on the processes tab.
Right-click on the Firefox browser.
Click on the "priority" tab.
Set the priority to lower.
When you have multiple applications competing for processor time, this will make Firefox yield time to more applications.
Bookmarking thread.