Make sure that you do not run Firefox in (permanent) Private Browsing mode (Never Remember History).

• https://support.mozilla.org/kb/Private+Browsing

• Tools > Options > Privacy > Firefox will: "Use custom settings for history"
• Deselect: [ ] "Always use private browsing mode"

In case you are using "Clear history when Firefox closes":

• do not clear the Browsing History

• Tools > Options > Privacy > Firefox will: "Use custom settings for history": [X] "Clear history when Firefox closes" > Settings
• https://support.mozilla.org/kb/Clear+Recent+History

Note that clearing "Site Preferences" clears all exceptions for cookies, images, pop-up windows, software installation, and passwords.

Thank you but I checked it; Under Privacy I have Remember history. I checked all the recommendations from the posts you refer to and I have them set correctly in all Tabs under Options

You can check the browser.sessionstore.* prefs on the about:config page and reset user set (bold) browser.sessionstore.*prefs to the default value via the right-click context menu to make sure that Session Restore is enabled and working properly.

• http://kb.mozillazine.org/Session_Restore
• https://support.mozilla.org/kb/restore-previous-session

• http://kb.mozillazine.org/about:config

If you use cleanup software like CCleaner then check the settings for the Firefox application.

Thanks but which one, please? I reset browser.sessionstore. restore_on_demand to true? It was the only one bold. But it did not help. Is this the right one? Do I need to restart Firefox now?

The browser.sessionstore.restore_on_demand pref is the setting in Options > Tabs: Don’t load tabs until selected.

You can use this button to go to the Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder)

Thank you very much. I entered the Profile but am unsure what to do in there. I do not see browser.session.store. there. I will look at it again tomorrow and will let you know.

You won't find browser.session.store in the Firefox profile folder, but you can check if you have any sessionstore files.

You can try to create a new profile as a test to check if your current profile is causing the problems.

See "Creating a profile":

• https://support.mozilla.org/kb/profile-manager-create-and-remove-firefox-profiles
• http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Profile_issues

If the new profile works then you can transfer some files from an existing profile to the new profile, but be cautious not to copy corrupted files to avoid carrying over the problem.

• http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox

I lost too many saved pages and bookmarks because the Restore Previous Sessioin option disappeared from my home page and was grayed out under history. If you're not computer savvy and the complicated suggested fixes are over your head or don't work, save all your Firefox bookmarks to Google Chrome. Delete Firefox and start using Google Chrome. I have never had this problem with Google Chrome.

Just happened to me after a system crash. I back up systemstore.js nightly. I retrieved the latest version but it appears to have been corrupted. Fortunately, the previous version worked. Lesson: Back up systemstore.js routinely and use a backup system that keeps multiple versions. Unfortunately, this won't help you get your session back if you haven't been backing up this folder. (On my Win7 system the file is in C:\Users\[username]\AppData\Roaming\Mozilla\Firefox\Profiles\[folder])

If the sessionstore.js file isn't very large then you can try to validate the file online.

• JSON Formatter & Validator: http://jsonformatter.curiousconcept.com/
• JSONLint - The JSON Validator: http://jsonlint.com/