First, there is occasionally a problem after Windows updates that when Windows starts up, Firefox has only limited permissions to access existing data. This usually is resolved by shutting down and restarting Windows. So I suggest trying that first.

Next, did Firefox perform a Refresh? Please check your desktop for a folder named Old Firefox Data, and if you find it, review the semi-randomly-named folder(s) inside to see whether they have current data that could be your missing settings. Anything there? Detailed steps for recovery to follow if applicable.

Restart resulted in my original home page being displayed, but history was only from today and bookmarks are vacant. I only have an Old Firefox set from several years ago, nothing from today.

Thanks so far

Could you set Windows to show hidden files and folders, and then do a search of your entire C:\Users\ folder for any folders named bookmarkbackups to see whether that uncovers any other profile folders?

http://windows.microsoft.com/en-us/windows/show-hidden-files

I've already done that. Just now, I copied a profile from my backup set into the proper Firefox folder, renamed it and tried a restart. It worked! Data were a little old, but still. Further, it seemed to open in the prior version of Firefox (I will recheck that). So, I think I'm okay for now, but the question remains as to what happened; what log do you wish me to access so I can send data to you? I'm also doing a secondary virus scan. There was no Windows update in progress; they either occur in the middle of the night or I do them manually.

Old profile starts up in 46.0

How different were the profile names, or was it nameX and nameX + a long string of numbers?

It's hard to guess why an entire profile would vanish without a trace just because of a system crash. I have heard of users who had a Firefox Refresh run and then shortly thereafter they ran a Windows system restore which removed the lost profile folder from the Old Firefox Data folder before a new restore point was made. I don't think that's a recoverable scenario, but I don't know if that's relevant here.

The one I restored from had the long string of numbers after a plus sign. The new "default" one created today had a relatively short alphanumeric string, preceded by "default". My prior two profiles had assigned names, but none appeared today either in my backup or anyware on the computer.

What if the profile didn't vanish but for some reason wasn't displayed in profile manager? There were two other long ones in the restored file; I chose the most recent. Historically, I have had two profiles plus a default. I went searching by long name, but found no others, though, in trying out this idea. So, it's not on my machine by its prior ID; I really do think it just vanished. I'm certainly open to trying to help you solve the mystery. I think I asked before if there's an event log you'd like--just let me know.

I went to HS in the Bronx with a guy named Scher, and have met a few others over the years. Small names-world.

cf

The Profile Manager displays what it finds in the profiles.ini file here:

C:\Users\username\AppData\Roaming\Mozilla\Firefox

There may well be "orphaned" profiles in the Profiles folder just below that folder, but I don't think Profile Manager will look for them unless the profiles.ini file is completely missing, and then it might take a survey of what is in there to rebuild the list. Honestly, I've never tested. But since you must have looked in there already and not seen any other profiles, I was just assuming it had disappeared.

There were other, much older, profiles, and since, demonstrably, profiles.ini is not missing, I'm not sure where else to go. So, unless you want more information, let's close up and just hope it doesn't happen again. Thanks for all your help. cf

Hmm, I think we've looked all the normal places, so that's probably the end of it for now.