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.