Recently heise security reported that tracking scripts are (under certain conditions) able to read saved email addresses and passwords (full report and video). One could now panic or disable this automatism to instead fill in saved form data with two clicks “manually”.

To disable the automatism you enter about:config in the address bar and confirm the warning to be careful.

On top of the list you search for signon.autofillForms which has a value of true by default. To switch that value to false you double-click the value true - and that’s it.