Hi,

Sorry about the issue. From the time frame it sounds like this happened probably after Firefox updates. A likely possibility could be security software interference. If so, to prevent this in the future you could try deleting all existing instances of Firefox and its related processes/files in all the different configuration modules/areas of the security software like process monitoring, sub/spawned process control, virtualization, HIPS etc., in addition to the security software's main configuration panel. Instead create new fully allowed/trusted rules for Firefox + its related processes in all the different modules/panels - a genuine/original Firefox installer as well as all the installed EXEs (Application) and DLLs (Application extension) are digitally signed by Mozilla Corporation (right-click > Properties > Digital Signatures). Even otherwise, some security software may also ask again when Firefox and/or its sub processes are started, and you may be able to allow/trust at that time. Please see this.

AV/Firewalls Configuration

The Avast forum and the respective forums for other security software would also be helpful.

If you still have the Firefox session open, you can try to Restore Previous Session via History (Alt + S).

If the Windows version includes the Previous Versions please try this: open the profile folder via Help (Alt + H) > Troubleshooting Information > Show Folder and exit Firefox. You can then right-click sessionstore.js which holds the tabs, or on an empty area of the folder > Restore previous versions and try to restore different versions, start Firefox each time and check.

Firefox Profile Folder & Files

Unfortunately Windows System Restore would not be able to restore the sessionstore.js file which stores the tabs.

Quote: Unfortunately Windows System Restore would not be able to restore the sessionstore.js file which stores the tabs.

This may not be true because the .js file extension may be a supported file extension for system restore.
However it is not recommended to use system restore as it usually doesn't restore all files, but only some and thus can cause inconsistencies.

You can try if you can restore a previous version: Right click: Properties > Previous Versions http://windows.microsoft.com/en-US/windows7/Previous-versions-of-files-frequently-asked-questions

In case you are using "Clear history when Firefox closes":

• do not clear the "Browsing History"

• Tools > Options > Privacy: History: [X] Clear history when Firefox closes > Settings
• https://support.mozilla.org/kb/Clear+Recent+History

Note that clearing "Site Preferences" clears all exceptions for cookies, images, pop-up windows, software installation, and passwords.

If you use cleanup software like CCleaner then check the settings for the Firefox application.

@CorEl: I had already unchecked everything in both Firefox and Advanced System Care that would delete History, Tabs, or anything else a loooong time ago.

@Dumdidadida: I sincerely appreciate the time you took to write such a long and detailed answer, but you might as well have written the first paragraph in Swahili, because I can't understand a word of it.

And I can't restore any previous sessions because all the History has been wiped out too. So many grouped tabs that I'll never be able to find again, it's just heartbreaking. Not to mention deleterious to my business.

Avast claims to have had nothing to do with it, they were the first ones I wrote. And you can't get in touch with the Advanced System Care folks at all. This morning everything came up fine, as far as it had been when I turned it off last night, not the grouped tabs.

Thank you to all of you for your help, I guess I'm going to have to manually copy and paste anything I add to the grouped tabs onto a Word doc so that if this happens again, I can manually get everything back using that.

Color me distressed and disgusted. But appreciative of all the help everyone has shown, thank you.