Did you try to open a new window to see if that makes all tabs visible?

cor-el said

Did you try to open a new window to see if that makes all tabs visible?

Not entirely sure what you mean.

Closing the browser & re-opening it doesn't resolve it. Opening a tab in a 2nd window doesn't resolve it. Yet the 2nd window doesn't experience the problem, only the first window does.

So, if the second window works then close the not working window and close and restart Firefox to see if it keeps working.

You can check for problems with the sessionstore.js file in the profile folder that stores session data. When sessionstore.js is deleted you lose pinned tabs and Tab Groups and open tabs and you will have to recreate them, so if necessary make a note or bookmark them.

You can use this button to go to the current Firefox profile folder:

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

@cor-el - Thank you for your help. Nice to know you're here to help.

I found the source of the problem. It's related to an AddOn.

I resolved it by going into about:config and setting an install location related to the AddOn.

You're welcome.

I have marked your above reply as the "Chosen Solution".