Display Detailed Firefox Session Information

Another interesting feature provided by this add-on is the ability to delete stored session information …

Ever since I started using Firefox I have configured the web browser to load the previous browsing session. This was not overly comfortable in the beginning as all sites were loaded at once on browser start, but I was able to improve that first by using add-ons and then later on natively when Mozilla introduced what these add-ons offered natively in the browser. Mozilla improved Firefox’s session feature in Firefox 33 which the stable channel will be moved to next Tuesday. The improvement makes the session restore feature more resilient against corruption so that it is less likely that sessions cannot be restored on start.

Session files are stored by Firefox in the user’s profile folder. They contain information about the sites and windows that were open the time the browser was closed by the user but also information about closed windows and tabs.

Firefox stores information about up to 3 closed windows and 10 closed tabs by default. You can change the values of the preferences browser.sessionstore.max_tabs_undo and browser.sessionstore.max_windows_undo to change those values on about:config.

While it is certainly possible to manipulate session information right in session files, it is not suggested as errors can render the file unusable.


The new Firefox add-on about sessionstore introduces a new about:page to Firefox which displays detailed information about the tabs and windows kept in session store.

After you have installed the add-on you can load about:sessionstore right away to display those information.  The page lists the following information:

  • Information about the size of the sessionstore file on disk, timings and session related settings such as the interval in which sessions are saved or how many closed tabs and windows are kept in it for the browser’s undo feature.
  • Detailed information about each open and closed window, and open and closed tabs.
  • For windows, this includes the size on disk, the number of open and closed tabs and cookies.
  • For tabs,it includes the page title, the size on disk, and storage, form and history information.
  • Information about closed windows and tabs.

One interesting feature, besides displaying all those information on a single screen, is the option to delete closed tabs and closed windows information right on that page.

If you want Firefox to “forget” about previously opened tabs and windows, this is probably the most comfortable option to do so. A click on one of the forget buttons — one for tabs and one for windows — removes the information right away from the session file and the page is refreshed with the new information.

There are other methods to delete those information but none is as elegant as the one provided by the extension.

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