Also Firefox has bad memory leakage issues. That's why it's best to restart it every now and then, if you leave it open permanently, eventually it owns all the RAM on the machine.
If you have some sort of memory issue on your machine I recommend this fix:
CleanMem by PcWinTech.com
Set this to run every 15 minutes, it's a background RAM fixer but perhaps more importantly it cleans the file cache, which for some reason is a problem area on some systems and can make a big difference.
It's true that some sites are worse than others, but in the end it's a software and RAM issue on the PC. You can tell how well your PC is set up by going to this site, which is very poorly run and one of the worst in the world for trash Javascript and Flash issues:
www.telegraph.co.uk
Browse
through 10 or 12 pages on that site with 3 or 4 tabs open, if your browser still runs OK then your machine is pretty much bulletproof. If the browser locks then depending on how early that happens, you know you need more grunt somewhere.
You'll see fairly soon that compared to that site, ECF is a speed machine... It's true that ECF is slower than it should be but we have a bunch of fixes for that coming up soon.