I have no chance to delete the hook at "Cookies and website datas will be deleted if I close firefox". I am using private modus but I have no idea where I can change this modus.
If you are using Private Browsing, all such data is stored in temporary files that are removed when the browser is closed.
You can check that you do not run Firefox in permanent Private Browsing mode (Always use Private Browsing mode; Never Remember History).
"Never Remember History" means that Private Browsing is active and "Always use private browsing mode" has a checkmark.
- Options/Preferences -> Privacy & Security
Firefox will: "Use custom settings for history"
- remove checkmark: [ ] "Always use Private Browsing mode"