Users who have tabs open in multiple Firefox windows can drag and drop tabs across separate browser windows. Using this technique you can consolidate all of your tabs into a single window. Keeping all of your tabs will make Firefox (and the rest of your system slower) but also let you make the most of a the “restore session” feature.
Session restores bring up all of the tabs you had open in case of a computer/program crash, or you close the window and later chance your mind.