Tabs are a great feature. For convenience, one might have twenty or more open at once. However, depending on the current activity, one might want a completely different set of tabs open, with the ability to switch between sets of tabs to suit different activities through the day. Firefox doesn't readily support this. Furthermore, after rebuilding a PC and installing Firefox, bookmarks may be easily transferred from a previous backup, but not the open tabs list (it is possible by copying over the relevant files, but undocumented and messy). Moreover, it is not unheard of for one's open tabs to go missing, resulting from an automatic update, or some other event.
Firefox has the ability to save and restore bookmarks, even import bookmarks from another browser, but not so for open tabs. Being able to save and switch between various tab sets should be a user-friendly and convenient option.
This request could be satisfied by providing options to (1) save the currently open tabs to a user-specified file, and (2) load tabs from a previously written file, optionally closing the tabs already open. The tab sequence should be preserved upon loading. There might still be a better, more convenient implementation.