Firefox uses too much resource. More than 600M! (10 tabs and 11 add-ons)
There is an addon for firefox that can help lower memory resources. It is called MemoryFox.
I second Dave on disabling the addons. In fact less addons you have the Faster your Firefox is. Once you get rid of unwanted addons check out following MakeUseOf article
The Ultimate Guide To Speeding Up Firefox 3.5
You should disable all of your addons and then enable them one by one to determine what is causing your memory leak.
You can use the same process with your tabs. Since you have different content loaded at different times, it is going to be hard to narrow down anything using the tabs. But if a particular site, and a flash plugin for example, causes a lot of memory usage, you can figure it out using this manner.