Sometimes Firefox is not able to update your session history file and starts a new one. In this case, during your session, everything looks normal. However, when Firefox starts up again, it has no record of creating a different sessionstore file and just restores the previous one. Let's check for that problem.

Open your current Firefox settings (AKA Firefox profile) folder using

Help > Troubleshooting Information > "Show Folder" button

Scroll down and look for files starting with sessionstore. Normally you will see two or three files: sessionstore.js (with a current timestamp), sessionstore.bak (with a timestamp corresponding to the last time you started Firefox), and a third file with a coded date created the last time you updated Firefox.

Note: If you find a file named sessionstore but not sessionstore.js, Windows may be hiding file extensions from you. To work with files as accurately as possible, I suggest unhiding them. This Microsoft support article has the steps: http://support.microsoft.com/kb/865219.

If you see files with names like sessionstore-1.js, then Firefox was unable to update sessionstore.js for some reason. We'll need to clean that up manually.

Leaving that window open, switch back to Firefox and Exit

Pause while Firefox finishes its cleanup, then rename sessionstore.js to sessionstore.old. If you need it again, you can rename it back and Firefox will find it at the next startup.

Next, look for the most up-to-date numbered sessionstore file and rename it to sessionstore.js. (If there are other numbered files, you can delete those or put them out of the way somewhere if you want them for possible future restoration.)

When you start Firefox again, if you have it set to restore your previous session, it should restore the latest one. Or you can use History > Restore Previous Session.

Any luck?

By the way, Firefox's default setting is to update the session history file every 15 seconds if you have opened or closed tabs or windows, or loaded a different page. If you do not see ANY recently updated sessionstore files, something else may be going wrong with Firefox's saving of your tab and window changes.

I'm sorry I took so long with a response, but I checked my folder and the most recent up-to-date numbered sessionstore file dates from February 20, 2014. Does this mean anything?

Are there any recently updated sessionstore files in the folder at all?

Could you check for any customized (non-default or "user set") settings here among the settings whose names start with browser.sessionstore:

(1) In a new tab, type or paste about:config in the address bar and press Enter. Click the button promising to be careful.

(2) In the search box above the list, type or paste sessions and pause while the list is filtered.

We could comment on ones that you find, or you could right-click > Reset them to their default values.

Note that Session Restore usually work better with the startup setting "Show my windows and tabs from last time".