Hint: if you don't want to apply safe mode options to your usual Firefox working environment, use Profile Manager to create a separate working environment.
Some problems with Firefox are caused by extensions or themes. This article will help you determine whether an extension or theme is causing your problem and, if it is, how to make Firefox run normally again.
[ ] Don't ask at startup
- without a tick in the check box - is preferred.
(If you check that particular box, you may find it less easy to use your default profile.)
Firefox prompts you for the profile to use each time you start Firefox