The following relates only to session history (windows and tabs open in your last session). The loss of browsing history and bookmarks may indicate a different approach. Next post addresses that.

First, could you check your History menu for any recently closed windows that could contain your missing tabs? If that doesn't yield what you're looking for...

At startup, Firefox 48 normally will look for sessionstore.js in the main level of your profile folder. Only if that is missing should it then look for recovery.js in the sessionstore-backups folder. And only if that is missing should it try to find recovery.bak or previous.js. As far as I know, other files in the sessionstore-backups folder would not be used automatically by session restore.

So let's try this:

(1) From inside Firefox, open your current Firefox settings (AKA Firefox profile) folder using either

• "3-bar" menu button > "?" button > 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 "Show in Finder" button. This should launch a Finder window listing the various files and folders in your profile.

(2) Leaving that window open, switch back to Firefox and Quit, either:

• "3-bar" menu button > "power" button
• (menu bar) Firefox > Quit Firefox

Pause while Firefox finishes its cleanup -- these changes should occur:

• recovery.js is moved from the sessionstore-backups folder to the main level of the profile folder and renamed to sessionstore.js
• recovery.bak is deleted from sessionstore-backups

(3) Once Firefox has stopped updating things, while viewing the main level of the profile folder, rename sessionstore.js to sessionstore-old.js (or remove it if you're confident it's useless)

(4) Copy the recovery.js file you want to restore to the sessionstore-backups folder

(5) Start Firefox and it should use the swapped in recovery.js file and restore whatever was in the substitute file; if you do not have Firefox set to restore your previous windows and tabs automatically, use History > Restore Previous Session

Any luck?

Okay, more generally, what kicked off this process? If you haven't shut down and restarted your Mac since this began, please try that first.

The two most likely scenarios for a complete loss of profile data are:

(1) A Firefox Refresh started but the migration of data from your old profile to your new profile was incomplete (this can occur if a Refresh hung or was killed or if a bug caused it to fail), or

(2) Your old profile was locked, so Firefox ignored it and created a new one

Here's how to investigate those two possibilities:

Did Firefox perform a Refresh?

Unless it was interrupted in progress, you should find a folder on your desktop named Old Firefox Data. Inside would be a semi-randomly-named folder with freshly updated files. Do you see anything like that?

If not...

Does the Profile Manager list your immediately previous profile folder?

To check on that, inside Firefox, type or paste about:profiles in the address bar and press Enter/Return to load it.

This page should list all the profiles Firefox is currently aware of. If you see a profile that is not currently set as default that looks promising, scroll down to it and click the Set as default profile button below that profile, then scroll back up and click the Restart normally button. (There are some other buttons, but I think those are still "under construction" so please ignore them.)

Firefox should exit and then start up using the other profile. Is that the right one?

If it's not there...

Did Firefox orphan your old profile folder?

Here's how to check whether there are additional profile folders in the standard location in addition to the one you're using. Start by opening your current Firefox profile folder using either:

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

In the first table on the page, click:

* Windows: the "Open Folder" button, to launch a new window listing various files and folders in Windows Explorer.

* Mac: the "Show in Finder" button, to launch a new window listing various files and folders in Finder.

In the Windows Explorer address bar, click the capitalized word Profiles to move up one level in the folder hierarchy. Mac users: please do what you need to do in Finder to check up one level.

More than one folder here by any chance? Do the files in those folders look like they were last updated around the time the problem began?

If you've got any promising looking folders, we can discuss next steps to recover use of them.

Thank you for the reply. First item: ALL history is gone, "restore previous session", "recently closed tabs", and "recently closed windows" have all remained greyed-out since this occurred, and "show all history" yields an empty library window, downloads and all. Following your sequence prompted different results for me; after quitting FF, "recovery.js" does not move from the "sessionstore-backups" folder, and "recovery.bak" also remains in that folder. Meanwhile, no " seessionstore.js" file apppears in the profile folder. The profile folder, btw, apppears to be a randomly named .default suffix, in case that indicates anything.... yet the files appear to contain the history. Meanwhile, the "recovery.js" file has been in the "sessionstore-backups" folder all along. I began to skim your 2nd reply; I have restarted the Mac, to see if it would offer any change, and also had attempted a safe FF start. I should mention that if I recall correctly, I seem to remember the OS taking longer to start than normal, as if it was having trouble on the power on self-tests.

For your second reply- no FF folder appears on my desktop. The profile manager lists 2 profiles under default - the root directory in the "Application support" folder in the library, and a local directory in the "Caches" folder in the library folder. The latter profile folder, however, contains no profile. Thanks for the help.

Firefox splits the profile folder over two locations.

One is the main profile folder with your personal data in a profile folder with a random name in the hidden "~/Library/Application Support/Firefox/Profiles" location.

The other location is the folder with the same random name in "~/Library/Caches/Firefox/Profiles/" is used for the disk cache and other temporary data. See the about:cache page for the location of the latter.

I think that you also have the about:profiles page in Firefox 48, so you can find buttons on that page to go to the profile folder.

Try to create a new profile to test if your current profile is causing the problem.

See "Creating a profile":

• https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles
• http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Profile_issues

If the new profile works then you can transfer files from a previously used profile to the new profile, but be cautious not to copy corrupted files to avoid carrying over problems.

• http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox

If your Firefox does not create a sessionstore.js file when you Quit, it might be set not to save history or to use private browsing automatically.

As an alternate approach, if you can adapt the steps to Mac, this thread has a couple methods for extracting the URLs from your old session history files: https://support.mozilla.org/questions/1164558