I'm sorry, the "Forget" feature has this terrible behavior and I am not aware of any workaround for it or quick recovery from it. I did file a bug report, but it is being treated as a feature request, so it isn't getting immediate attention: https://bugzilla.mozilla.org/show_bug.cgi?id=1497731

If you don't have a recent (enough) backup of your Firefox profile folder, it's possible Windows took a snapshot during a system update. Don't use system restore! That typically causes more problems.

For Windows 7, you have two options:

(A) Previous Versions Tab (not present in Win8, 8.1, or 10)

While Firefox is running: open your current Firefox settings (AKA Firefox profile) folder using either

• "3-bar" menu button > "?" Help > Troubleshooting Information
• (menu bar) Help > Troubleshooting Information
• type or paste about:support in the address bar and press Enter

In the first table on the page, click the "Open Folder" button. In your profile folder, double-click into the sessionstore-backups folder.

Right-click the recovery.jsonlz4 file, then click Properties, then click the Previous Versions tab. This should search recent restore points for copies of this file at the identical location (i.e., within the same profile).

If you find any promising (i.e., recent) candidates, you can use the Copy... button to save them to a safe location for examination ("scrounging" using https://www.jeffersonscher.com/ffu/scrounger.html).

If you have a previous.jsonlz4 file, check that one, too.

This method doesn't work with files that are no longer present in the folder, so for those:

(B) Restore Point Explorers

There are several programs you can use to export files from restore points. Here's the rundown:

(1) Set Windows to show hidden files and folders:

https://support.microsoft.com/en-us/help/14201/windows-show-hidden-files

(2) Download and run one of these programs:

• ShadowExplorer: http://www.shadowexplorer.com/
• System Restore Explorer: http://nicbedford.co.uk/software/systemrestoreexplorer/

Within the most recent restore point shown in the program, you can explore along this path to see whether you can find a timely shadow copy of your profile:

\Users\your-user-name\AppData\Roaming\Mozilla\Firefox\Profiles\

(3) Within your profile folder, check the sessionstore-backups folder for any useful looking files and Export them to a safe location.

Then you can scrounge them to see whether they have what you need.

Thank you very much. I was able to solve it with Previous versions of the file [recover.jsonlz4], fortunately there was a version of a few days ago. I wasn't aware of this bug clearing all the data in sessionstore-backup folder before this, I'll be careful not to use 'clear recent history' or 'Forget' in the future, at least until it's fixed.

The Forget button is meant as a 'panic' button and all open windows are always closed. You should see a warning about this.

• https://support.mozilla.org/en-US/kb/forget-button-quickly-delete-your-browsing-history

Hmm no I used the 'Clear recent history' of last one hour, not the Forget feature, and I lost my closed (unopened) windows not the open ones. I've been using that with no problem until now. Thanks for the warning, though I haven't tried using Forget feature and probably won't as it was pointed out by jscher that it is bugged.