You need to locate the profiles.ini file and delete this file or use the Profile Manager to create a new profile.

See:

• https://support.mozilla.org/kb/how-run-firefox-when-profile-missing-inaccessible

This is usually caused by a problem with the profiles.ini file.

You can use one of these to make Firefox create a new default profile or recover an existing profile:

• Delete the profiles.ini file to make Firefox create a new (default) profile
• Use the Profile Manager to create a new profile
  If you still have an existing profile then click "Choose Folder" in the Profile Manager and browse to the location of a lost profile to recover this profile
  https://support.mozilla.org/kb/Managing+profiles

You should find the profiles.ini file in this hidden location:

• Windows: %AppData%\Mozilla\Firefox\
  *C:\Users\<user>\AppData\Roaming\Mozilla\Firefox\
• Mac: ~/Library/Application Support/Firefox/
• Linux: ~/.mozilla/firefox/

If clearing the profiles.ini file isn't working then also check the location of the cache files.

• Windows: *C:\Users\<user>\AppData\Local\Mozilla\Firefox\Profiles\<profile>\
• Mac: ~/Library/Caches/Firefox/Profiles/
• Linux:~/.cache/mozilla/firefox/

• http://kb.mozillazine.org/Profile_Manager
• https://support.mozilla.org/kb/Managing+profiles
• http://kb.mozillazine.org/Profile_folder_-_Firefox

1. 1 Do you need to reinstall firefox after this step?

1. 2 Keep Add On's or Reinstall from scratch

1. 3 Sync - (Just Sync Tabs, Bookmarks & Passwords) Nothing Else?

What issue do you still have?

Is that the problem with the profile folder (profiles.ini file)?

Can you list the content of the profiles.ini file?

See my reply above for the location of this file.

Is the profile folder that is marked with Default=1 in the file present?

Are there any profile folders found in either location used by Firefox?

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

• http://kb.mozillazine.org/Profile_folder_-_Firefox

Again... after all of that.. is it best to

1 Do you need to reinstall Firefox after this step?

2 Keep Add On's or Reinstall from scratch

3 Sync - (Just Sync Tabs, Bookmarks & Passwords) Nothing Else?

If there are issues with crashing then you should first try out a clean as little as possible modified profile folder. If that works then try restoring a few files to see if it still work. The bookmarks are probably best restored from a JSON backup in the bookmarkbackups folder.

• Bookmarks > Show All Bookmarks > Import & Backup > Restore > Choose

Maybe it is best to wait a few days before setting up Sync because Sync can cause issues. If there is still data on the Sync server then maybe you should reset this data by changing the password of the sync account.

Create a new profile as a test to check if your current profile is causing the problem.

See "Creating a profile":

• https://support.mozilla.org/kb/profile-manager-create-and-remove-firefox-profiles
• http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Profile_issues

If the new profile works then you can transfer files from a previously used profile to the new profile, but be cautious not to copy corrupted files to avoid carrying over problems.

• http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox