This was fixed for extension-free firefox. Today this is fixed, for extensions as well.

What was happening was that extensions had references to dom-nodes of a page, that prevented the page from being unloaded when you close the tab. Unless of course, the extendion author bothered to fix this from their side. Now, an extension has essentially "weak references" to dom nodes. (think: symlink)

I second this. On Ubuntu 12.04 Unity crashes after approximately 20 days of uptime and that's Almost the only time I restart Firefox. I regularly have a single session open for weeks.