Simply sign out of your Firefox Account via about:preferences >> Firefox Account >> Sign Out.

Cant get to firefox sign out? I want to remove my account from this computer?

To delete all of the data associated with your Firefox sync account:

1.Go to Firefox Accounts 2. Login using the information associated with your sync account 3. Delete your account 4. Signup for a new sync account using the Firefox Accounts website or directly from Firefox

Warning: This cannot be undone and will result in the loss of your sync data