This Kind Of Thing is why I'm now using Chrome. I'm personally using official Google Chrome, but you can get chromium from the PPA which is an open source project.
For some reason (some specific reasons actually, but is probably a bit out of scope for the discussion here) Firefox has always been somewhat problematic with laggy GUI issues on GNU/Linux-based systems. It's been improved over the years, and isn't very noticeable when you have a small number of tabs or first start, but it quickly gets out of hand in my experience, where I have several tabs open to Google applications and other "Web 2.0" web app kinds of things. I'm the kind of person who doesn't like to reboot or quit my browser and restart unless I have to, so the less leaky the browser, and the better it can recover from bad pages, the better.
I wish I had a magic wand, but "switch to Chrome or chromium" is probably your best bet, honestly. It's still not perfect, but at the very least the UI is lightning-fast compared to Firefox and you can get an idea of what is eating all your memory and cpu by pressing shift+esc which lets you see a breakdown of what is in use by each tab.
It also helps to install Flashblock Chrome or something similar to keep ads and such from taking over your system resources.