I have a need to use Firefox with multiple independent sessions, due to some websites with problematic path-dependent navigation. Firefox's "Private Browsing" option seems to allow multiple windows and tabs, but they are all tied to the same session. In the current configuration, there seems to be exactly one session for all of the non-private windows and tabs, and exactly one other session for all of the private windows and tabs. This is a problem because sometimes I need a variety of windows each with a different session, and don't want to be limited to only two of them.
Are there any options in Firefox or add-ons to support this?