You may have corrupt sessionstore 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.

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

This worked!

Two things: the button mentioned as "Show Folder" is actually "Open Folder", and I presume "Locate the above file" means the sessionstore file in that Open request.

Thanks for the solution.

...tom

Sadly, after working a few times, the browser has reverted back to the initial complaint. ...tom

https://support.mozilla.org/en-US/kb/how-to-fix-preferences-wont-save

Note: Some software, like Advanced SystemCare with Surfing Protection, can protect files in the Firefox profile folder against changes. If you have such software then check the settings or uninstall this software.

Refresh and safe mode don't alleviate the problem. Preferences are apparently saved because my choices display properly. This has only started with 56.0 and I've been using Firefox for many years.

Thanks for the suggestions.

Update: I installed 55.0.3 and the problem went away (for now at least).

Perhaps you can take a few minutes to file a bug report; https://bugzilla.mozilla.org/ This is in case others get the same problem.

Are you using external cleanup software like CCleaner to clear session data?

It is possible that Firefox isn't closing properly and considers this as a crash.

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

Also make sure to wait long enough for Firefox to close completely before powering down Windows.