- Delete %appdata%/Mozilla/Firefox/profiles.ini file
 - Delete the whole %appdata%/Mozilla/Firefox folder
 - Re-install firefox
 - Delete the %appdata%/Mozilla folder, delete C:\ProgramFiles\Mozilla Firefox folder and re-install
 - Remove old profile and add new one using "firefox.exe -P"
 - Repeat some of the above 5x

 - I noticed Kaspersky antivirus was demanding restart after its own update.
 - Firefox error said "inacsessible", and antivirus frequently locks out our files during development.
 - Kaspersky forces its plugins into browsers, so I gave restarting it a try.
 - The Firefox works after reboot with no errors even with the original (restored) contents of the %appdata%/Mozilla folder.

 - The Kaspersky automatically installs its extension into browsers (even after manually removing or disabling them). 
 - I suspect that the updates/installs somehow conflicted.

Hi! After update to 56.0, I was getting multiple following messages when starting and/or using Firefox. Eventually, all my bookmarks loaded without problem, but the error windows appeared consistently: "Your Firefox profile cannot be loaded. It may be missing or inaccessible." Attempted solutions (with no success): - Delete %appdata%/Mozilla/Firefox/profiles.ini file - Delete the whole %appdata%/Mozilla/Firefox folder - Re-install firefox - Delete the %appdata%/Mozilla folder, delete C:\ProgramFiles\Mozilla Firefox folder and re-install - Remove old profile and add new one using "firefox.exe -P" - Repeat some of the above 5x Working solution: Reboot - I noticed Kaspersky antivirus was demanding restart after its own update. - Firefox error said "inacsessible", and antivirus frequently locks out our files during development. - Kaspersky forces its plugins into browsers, so I gave restarting it a try. - The Firefox works after reboot with no errors even with the original (restored) contents of the %appdata%/Mozilla folder. Suspected cause of the problem: - The Kaspersky automatically installs its extension into browsers (even after manually removing or disabling them). - I suspect that the updates/installs somehow conflicted. EDIT: Reformatting for better readability. Added missing info into "attempted" solutions.