Firefox includes an option for bookmarking all open tabs, but heavy users of tabs will find that this option is hardly enough. When you are researching a subject, the particular combination of tabs matters as much as the individual ones -- and, besides, selecting the tabs to open individually can be tedious if you are dealing with several dozen. And what happens if your session crashes before you have a chance to bookmark? You can address such concerns by installing Session Manager, a highly customizable add-on for preserving the state of the window after you close the browser.
Session Manager installs as an icon on Firefox's Navigation toolbar, so you won't see it after installation. To make it visible you need to right-click on the toolbar and click Customize, then drag Session Manager to the toolbar.