To save all open Websites/Tabs in Firefox befor closing it

Press Ctrl+Shift+D or go to Bookmarks and select "Bookmark all Tabs".
Name the Bookmark.

In the Create in or Folder Option U can select either  Bookmarks Menu OR  Bookmarks Toolbar .
 Click Add Bookmarks Button


This will save all the open tabs into one folder and you can resume surfing the next day by select "Open All in Tabs" from the bookmark folder.

* Tip: Name this folder something like "Sept 5" or "090510" so it becomes easy for you to guess the date from the folder name itself.