Hey, here is a cool new tip I just found out about. Say you have two Firefox Windows open with both having multiple tabs. You can drag a tab from one Firefox browser window to the next. Here is a better description of the cool tab tip from the source:
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.