We have an article that discusses several possible reasons for preferences not to be retained between sessions: How to fix preferences that won't save. Do any of those seem applicable to your system?

The article doesn't help. The files "prefs.js" and "prefs.js.moztmp" are nowhere to be found, there is no other file with a "prefs*" prefix at all, and no "invalidprefs" either. I've just been reminded that I couldn't run Firefox from PortableApps on this machine too, because of a missing profile.

Every profile folder should have a prefs.js file to store customized preferences. Even if you haven't customized the toolbars, new tab page, or settings yourself, as soon as you start running Firefox, it records information about how large your cache should be, whether you've seen the welcome page already, and numerous other details, to prefs.js.

It might help to set Windows not to hide things from you, such as file extensions and hidden folders:

• https://www.bleepingcomputer.com/tutorials/how-to-show-file-extensions-in-windows/
• https://support.microsoft.com/en-us/help/14201/windows-show-hidden-files

And try opening the profile folder through the "Open Folder" button on the Troubleshooting Information page if other methods do not work: Profiles - Where Firefox stores your bookmarks, passwords and other user data.

Thank you, jscher, for responding so quickly. I have not hidden extensions or folders fora good many years, so those are not issues. I assure you that the files you named are not in the profiles folder. There is just one folder nested inside the profiles folder all by itself, entitled "wszwqryx.default", and nothing else. Inside it are 13 folders and 40 separate files, none of which have "prefs" in the name. I tunneled most of the way down into the folders within folders within folders and found no "prefs". What's in there generally looks appropriate for a profiles folder, but nothing that you named. I will send screenshots if you think it'll help.

Is Firefox able to read/write other data to the folder such as:

• bookmarks and history entries in places.sqlite

• changes to displayed bars (e.g., Menu Bar and Bookmarks Toolbar on) and window sizes/positions in xulstore.json

• currently open and recently closed tabs/windows in sessionstore-backups\recovery.jsonlz4 and sessionstore-backups\recovery.baklz4

Note: the recovery files are removed during a normal shutdown, but should be updated as often as every 15 seconds during your Firefox sesssion.

I found all those files but can't open them directly. Firefox retained a test bookmark and the bookmarks bar across a shutdown and startup, but not a single setting.

Yeah, don't try to open those files, it was just a question of whether Firefox is able to write to the profile folder.

I don't know why Firefox isn't creating the prefs.js file, or whether the system or some third party software is somehow preventing Firefox from doing it. Baffling.

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder in the hidden 'AppData\Roaming" folder or use the about:profiles page to go .

• Help -> Troubleshooting Information -> Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox

• C:\Users\<user>\AppData\Roaming\Mozilla\Firefox\Profiles\<profile>\

Thank you, Cor-el. As I told Jscher, "I assure you that the files you named are not in the profiles folder. There is just one folder nested inside the profiles folder all by itself, entitled "wszwqryx.default", and nothing else. Inside it are 13 folders and 40 separate files, none of which have "prefs" in the name. I tunneled most of the way down into the folders within folders within folders and found no "prefs". What's in there generally looks appropriate for a profiles folder, but nothing that you named." I also searched the entire hard drive - no joy. I do have a "channel-prefs.js" at the end of the path "C:\Program Files\Mozilla Firefox\defaults\pref". I have nothing that approaches "prefs.js.moztmp". Incidentally, I had the same issue running Firefox Portable from the PortableApps suite on a flash drive, but I was too busy fighting another issue on the machine to delve into it.

What files are in the profile folder?

Can you attach a screenshot?

• https://support.mozilla.org/en-US/kb/how-do-i-create-screenshot-my-problem
• use a compressed image type like PNG or JPG to save the screenshot

I'm still not finding much in the way of solutions from searches. Here's the screenshot:

I just tried the profilewizard with no result. I found a profile.ini to delete but that did nothing either.

I'm still seeking solutions from the 'Net, but I can't find any more that I haven't tried. Overlay reinstall didn't help any. I tried a complete wipe and reinstall - same thing. The fact that running Firefox Portable from a flash drive had the exact same result makes me think it's something local to my machine. Any ideas?

What if you try to right-click > Save Link As on a .js file such as:

https://raw.githubusercontent.com/jscher2000/switch-to-previous-active-tab/master/popup.js

Does your system let you save that?

I want to thank you, jscher and cor-el, for your assistance. I did finally find a solution - I dug and dug and found out there is a page where Firefox can be reset. It worked. I haven't tried it with Portable Firefox yet, but I will soon and will post the result as soon as I do.