New feature: save and load tab sets

When firefox starts-up it restores my previously open tabs, saved on last exit. To extend the restore-tabs-on-startup facility, I would like the ability to save (and later open) a tab set by name using e.g. File-> menu options. This extended facility would give the ability to relate many different sets of tabs, for example when I'm working on 2 or 3 different projects and want to associate several web pages with each, or for example to open a tab set of my favourite related web-sites. Currently I remember sets of tabs by organising my bookmarks into folders, and File->new_tab and open the folder contents one-by-one.