Hourra !
Je vais pouvoir copier coller mon adresse e-mail quand on me la demande deux fois !!

You need to use about:config which is a feature of Mozilla applications which lists application settings (known as preferences) that are read from the profile files prefs.js and user.js, and from application defaults. The following hack enables the password paste. In Firefox a type about:config in the Location Bar.

If you see a page with the warning message, This might void your warranty!, click the button labeled “I’ll be careful, I promise!“, to continue. > Type the following in search bar:
dom.event.clipboardevents.enabled

And set dom.event.clipboardevents.enabled value to false by double clicking on it.

Merci Firefox !