After reading a post here: http://www.weblogs.mozillazine.org/ben/archives/009749.html
Ben, the lead Firefox engineer posted on why firefox consumes memory at an increasing rate when browsing the web with multiple tabs. Turns out firefox caches your page history (up to 8 pages PER TAB) in memory to speed up back/forward page browsing. I've since disabled this feature and Firefox has been running at a nimble 54MB of ram for a little while now with 10 tabs and multiple pages browsed in each.
in your address bar type in: about:config
Then navigate to this setting: browser.sessionhistory.max_total_viewers
it defaults to -1 which uses the max cache, change that to 0 to disable the feature and you'll be sucking down less ram in no time. Restart the browser and you should be good to go. Your nimble firefox is back.