You may have corrupt sessionstore [v56] sessionstore.jsonlz4 file(s). Delete all sessionstore* files and the sessionstore-backups folder.

Type about:support<enter> in the address bar.

Under the page logo on the left side, you will see Application Basics. Under this find Profile Folder. To its right press the button Show Folder. This will open your file browser to the current Firefox profile. Now Close Firefox.

Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder

Linux: Under the page logo on the left side, you will see Application Basics. Under this find Profile Directory. To its right press the button Open Directory.

Locate the above file. Then rename or delete it. Restart Firefox.

Don't delete the files if you need to rescue any data from them, just move them out of the profile folder to some location where Firefox doesn't look for them. You can try to read out their contents using this tool: https://www.jeffersonscher.com/res/scrounger.html

In the address bar, type about:crashes<enter>. Note: If any reports do not have BP- in front of the numbers/letters, click it to submit them.

The crash report is several pages of data. We need the report numbers to see the whole report.

Using your mouse, mark the most recent 7 - 10 Submitted crash reports, and copy them. Now go to the reply box and paste them in.

For more help on crash reports, see; https://support.mozilla.org/en-US/kb/firefox-crashes-asking-support

Firstly, there aren't any files or directories with names matching "sessionstore*" in this profile directory. (At least not when FF isn't running. When FF is running, there is a directory called 'sessionstore-backup', but it gets deleted when I quit.)

Secondly, and more disturbingly, I started a new profile. New profiles seem to work in that, if I set them to remember session, it works. But, as soon as I sync-ed with my FF account, it broke session restore.

Thirdly, I did find a large number of reported and unreported crashes, but none of them were with the last month. I just cleared all of them, which is fine with me.

Shut down sync for now. What a few days. Any problem?

Ah, I think I know. I think clearly history on close is set. I thought I left history unchecked, but I guess not. Let me try this...

Type about:preferences#privacy<enter> in the address bar. The button next to History, select Use Custom Settings.

Turn off Always Use Private Browsing Mode Turn on Remember My Browsing And Download History At the bottom of the page, turn on Clear History When Firefox Closes. At the far right, press the Settings button. Turn on ONLY Cache and Form And Search History leaving the others off. and whatever else you want.

Use one of these to close Firefox if you are currently doing that by clicking the close X on the Firefox Title bar.

• "3-bar" menu button -> Exit (Power button)
• Windows: File -> Exit
• Mac: Firefox -> Quit Firefox
• Linux: File -> Quit

If you use "Clear history when Firefox closes" then try to disable this feature.

• 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