Bookmark or save multiple tabs
From MozillaZine Knowledge Base
Bookmark all open tabs
You can bookmark all open tabs into a new bookmark folder. Open the Add Bookmark dialog (Ctrl+D or "Bookmarks -> Bookmark This Page" or "Bookmark this page" in context menu). Check the "bookmark all tabs in a folder" checkbox. Change the name to be the name of the new bookmark folder you wish to create, and then select a parent folder. Finally, click OK.
A folder with bookmarks to all open tabs will be created. You can later reopen the tabs you saved by middle-clicking the folder or selecting the "Open in Tabs" menu item.
Save a browser session
If you wish to save a full browser session (with tab histories and so on) then you will need to use an extension such as SessionSaver.