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

Thank you, cor-el, but I never use “clear history,” and FF is still set to “show windows and tabs from last time” as always.

cor-el said

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

Did you check that you do not run Firefox in permanent Private Browsing mode (Always use Private Browsing mode; Never Remember History)?

• https://support.mozilla.org/en-US/kb/private-browsing-use-firefox-without-history

• Options/Preferences -> Privacy & Security
  Firefox will: "Use custom settings for history"
• remove checkmark: [ ] "Always use Private Browsing mode"

Thanks again, cor-el. No, I do not use private browsing mode, and I never changed any settings before this problem happened.

Do you have any idea of how I can solve my problem and somehow restore that last session and bring back all of those tabs?

You would need to look for sessionstore.jsonlz4 in the profile folder.

• ~/Library/Application Support/Firefox/Profiles/<profile>/
• https://support.mozilla.org/en-US/kb/recovering-important-data-from-an-old-profile

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.

• Help -> Troubleshooting Information -> Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox

Do you know how I can use Time Machine to restore this file so that it opens a FF session with those lost tabs? I've located this file from a few days ago, but don't know what to do with it so that I can restore that previous session.

You need to place the sessionstore.jsonlz4 from the Time Machine backup in the Firefox profile folder to replace the current sessionstore.jsonlz4 file. This file is only there when Firefox is closed completely (Firefox -> Quit Firefox)

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.

• Help -> Troubleshooting Information -> Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox

You can first inspect that sessionstore.jsonlz4 file to see if it has the content you are looking for.

• https://www.jeffersonscher.com/res/scrounger.html

Did you already check the "Recently closed windows" list? Sometimes users close windows individually instead of using Quit, and in that case, Firefox should show the three most recently closed windows on that menu after the session is restored.

HUGE THANKS to you, cor-el!! You're my hero—thank you for being so patient and walking me through this.

Your last set of instructions, especially the tidbit about Firefox having to be closed while restoring the sessionstore file, simply did the trick. I stopped overthinking it—I closed FF, restored the file through Time Machine, and did nothing else. I left the restored file alone instead of moving it to where I thought it belonged. Then I reopened FF and—BAM!—all of those tabs were back!!!

-))