I've been having the exact same issue. I tend to leave a relatively large number of windows/tabs open and Firefox is starting to edge towards unusability on a 4x3.2ghz + 16GB RAM system simply because its performance and memory consumption is interfering with other applications.
Though I really think most major browsers are somehow all simultaneously losing the thread. Websites are fundamentally just formatted text and extremely basic graphics, for the most part. It's not like I have 50 high end webgl demos running. Rendering text and basic graphics is not something that should be consuming anywhere remotely near the resources that have become typical. If javascript is the problem, then that should be dealt with and not simply accepted as the price of doing business. In many ways I feel like the mantra that 'premature optimization is the root of all evil' has gradually turned into 'any form of optimization short of making sure you don't run O(n) in O(n^2) is not a priority.' But like anybody who's worked on a project knows those placeholders that 'you'll get to later' end up being exactly what goes live. And when optimization is disregarded, you get text/graphics rendering struggling to perform on systems millions of times stronger than what sent us to the moon.
Though I really think most major browsers are somehow all simultaneously losing the thread. Websites are fundamentally just formatted text and extremely basic graphics, for the most part. It's not like I have 50 high end webgl demos running. Rendering text and basic graphics is not something that should be consuming anywhere remotely near the resources that have become typical. If javascript is the problem, then that should be dealt with and not simply accepted as the price of doing business. In many ways I feel like the mantra that 'premature optimization is the root of all evil' has gradually turned into 'any form of optimization short of making sure you don't run O(n) in O(n^2) is not a priority.' But like anybody who's worked on a project knows those placeholders that 'you'll get to later' end up being exactly what goes live. And when optimization is disregarded, you get text/graphics rendering struggling to perform on systems millions of times stronger than what sent us to the moon.