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

Try to use "Firefox/File > Exit" (Mac: "Firefox > Quit"; Linux: "File > Quit") to close Firefox if you are currently doing that by clicking the close X on the title bar.

It is possible that there is a problem with the files sessionstore.js and sessionstore.bak in the Firefox Profile Folder.

Delete the sessionstore.js file and possible sessionstore-##.js files with a number and sessionstore.bak in the Firefox Profile Folder.

• Help > Troubleshooting Information > Profile Directory: Open Containing Folder
• http://kb.mozillazine.org/Profile_folder_-_Firefox

Deleting sessionstore.js will cause App Tabs and Tab Groups and open and closed (undo) tabs to get lost, so you will have to create them again (make a note or bookmark them).

• http://kb.mozillazine.org/Multiple_profile_files_created

Thanks for your help and wisdom. I deleted the sessionstore.js and created new pin to app links. Worked well. Your the best, take tomorrow off.

You're welcome

Well I thought it was fixed. This morning I opened up FF and the pin as app tabs were missing.

Are you using Firefox (or File) > Exit Firefox to close Firefox and all open windows?

Try this extension:

• App Tab Initializer: https://addons.mozilla.org/firefox/addon/app-tab-initializer/

If possible, I would prefer to find out what approach I can follow - if I notice soon enough that the pinned and app tabs I expect are not present - to recover a recent snapshot of my pinned and open app tabs. It is a lot less satisfactory to be told: too bad, start over like this...

In my case, last night, I installed uTorrent including their toolbar. Without Firefox closing and after uTorrent 3.2 had finished installing, I saw that Firefox was still open, but all my pinned and app tabs that were open (before I installed uTorrent) had gone and only a single app tab was open.

Most often, my Firefox has been open for weeks and I have many (50-70) open app tabs in several tab groups and about ten pinned tabs. Even through a recent Windows mandatory security files update which rebooted my laptop automatically, my pinned and app tabs were preserved. So it is massively frustrating when this problem occurs without knowing a good strategy to RECOVER rather than to recreate from scratch.

I know - life is hard, then we die - sometimes starting over is really the only way, but it should be very much a last resort.

App tabs can disappear if you close windows in the wrong order, i.e. you first close the window with the App tabs and then notice that there is still another window open.
In that case you won't get the App tabs restored if you do not reopen the other window first.
The best way to avoid such issues is to always use "File > Exit" to close Firefox and all open windows at once.

I used the app-tab-initializer extension that you recommended. It seemed to work for a while, and then...it doubled everything. I had 2 app tabs per app so I disabled it. Pin as app is at best, broken. File > Exit may be the only way to go, but it's hard to do all the time. I've been closing windows with Alt+f4 for almost 2 decades now.

It's really not worth bothering about. When it works great. When it doesn't, I'll just add them again. Humbug12...good luck bro. I hope a fix comes around for you soon...50-70 tabs...yikes! I wouldn't want to walk a mile in your shoes.

Cor-el, thanks again for your time and input. Have a great weekend.

I was going to download the app until I saw the notation "author not verified"....

That is OK with extensions that are hosted on the Mozilla Add-ons site.
Extensions hosted on that site have been examined and are OK to install unless they show that they haven't been examined.
Almost all extensions will have an "author not verified" notice.