Hi, just for a Mac, you will make Windows users mad.... You can always check here to see what was fixed, added or security or all of them : https://www.mozilla.org/en-US/firefox/releases/

Please try a Refresh but Note that this does more than uninstalling and re-installing does. Normal uninstalling does not remove some preferences which may have become corrupt. Please Refresh

• https://support.mozilla.org/en-US/kb/export-firefox-bookmarks-to-backup-or-transfer
• https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles

then

• https://support.mozilla.org/en-US/kb/refresh-firefox-reset-add-ons-and-settings

TEST....... If no issues then Extensions which need to be added back in 1 at a time and tested ..... Or it is your Profile : Make a new one and test ...:

• https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles

If is your Profile :

• https://support.mozilla.org/en-US/kb/recovering-important-data-from-an-old-profile
• http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox

Note: Any customization will revert back to default, you will also need to reinstall Extensions.

Please let us know if this solved your issue or if need further assistance.

Thanks for your suggestions. I doubt whether any of these solutions will bring back the ~30 tabs in 4 windows that were not bookmarked, and now do not even show up in the history, which appears to have been wiped. I may be able to find the tabs from a computer backup when I return home (I am now overseas). Perhaps it could prevent future issues: i.e. restarts esp after "upgrades" not "losing" the details of the previous session from history or having the 'restore previous session' greyed out, but I am not confident of that. As mentioned, this has been a recurrent problem with Firefox, and has never ever happened with Safari. It should be a very basic functionality within any browser program, that should always work, and not become a problem with an upgrade.

In case you use "Clear history when Firefox closes" or otherwise clear history.

• do not clear the Browsing History

• Options/Preferences -> Privacy & Security -> Firefox will: "Use custom settings for history": [X] "Clear history when Firefox closes" -> Settings
• https://support.mozilla.org/en-US/kb/delete-browsing-search-download-history-firefox

• clearing "Site Preferences" clears exceptions for cookies, images, pop-up windows, and software installation and exception for password and other website specific data
• clearing "Cookies" will remove all selected cookies including cookies with an "Allow" exception you may want to keep

Make sure you close Firefox (Firefox -> Quit Firefox) with all windows still open.

Firefox uses the sessionstore.jsonlz4 file in the profile folder to store session data. The sessionstore.jsonlz4 file is only present when Firefox is closed. The sessionstore.jsonlz4 file is created from recovery.jsonlz4 when you close Firefox and is removed and copied to previous.jsonlz4 when you start Firefox to make is possible to restore the session at any time.

You will normally find these files in the sessionstore-backups folder:

• previous.jsonlz4 (cleanBackup: copy of sessionstore.jsonlz4 from previous session that was loaded successfully)
• recovery.jsonlz4 (latest version of sessionstore.jsonlz4 written during runtime)
• recovery.baklz4 (previous version of sessionstore.jsonlz4 written during runtime)
• upgrade.jsonlz4-<build_id> (backup created during an upgrade of Firefox)

You can copy a file from the sessionstore-backups folder to the main profile and rename the file to sessionstore.jsonlz4 to replace the current file (make sure to backup the current sessionstore.jsonlz4).

Firefox 56+ releases compress the files in the sessionstore-backups folder (mozlz4).