I used to use an extension called BarTab, which would allow me to delay the loading of a tab until I actually clicked it to visit it. So, when I'd start my browser and it would restore all my tabs from my last sessions, they wouldn't load and thus save me a lot of memory. Once I clicked on a tab, the page would actually load. This was wonderful, but BarTab hasn't officially supported a version of Firefox since 3.x, and today we're on 8. (I've been editing the extension to alter the version it supports, but that's fragile and not a good long term solution.)
Good news though! Firefox 8 now supports on-demand tab loading! I would post screenshots, but my interface is in German, so just follow these instructions if you need detail. Elsewise, just go to Preferences, General, and with "restore tabs from last session" selected as your default home page, check the "Don't load tabs until selected" box. Yay!