If Firefox is a bit slow to start up and fast to use up RAM due to tons of open tabs, you can turn on a quick setting in Firefox 8 to only load tabs one at a time, when you click on them.
Firefox 8 introduced a few different features, but one that slipped by us was a new BarTab-like feature that stops Firefox from auto-loading tabs. If you have Firefox set to open up your tabs from last time whenever you start it up, you probably notice it takes a while to start up and can start taking up RAM pretty quickly. To keep its memory usage low, just head into Firefox’s Options, go to General, and check the “Don’t load tabs until selected” box. From then on, it will only load tabs as you click on them. You’ll be less likely to get crashes, and it won’t take quite as long to start up and get working. You may have to wait for a second or two when you do switch to those tabs, but it’s a fair trade, depending on how many you usually have open at startup.
Changes to “Don’t Load Tabs” option [Firefox Extension Guru’s Blog]