Re: How can i import new cookie into the last version Firefox or UX ?
I am not sure I understand whether you are trying to create or edit a cookie, or merely log in to a site that requires a password and login name.
To simply delete or edit cookies use
Firefox Button -> Options -> | Privacy | History remove individual cookies
the option opens a dialogue, if a site then a cookie is selected the details are shown and are editable
individual cookies may also be deleted, there is a clickable button for deleting
If you need to do more sophisticated work, then because the cookies are now stored in a database, rather tan as individual files, probably using an add-on is the easiest method of manipulating them e.g. :