At the very beginning after installed Syncthing on Win10, Syncthing can automatically logged in after run, but I am not sure starting from when, it need me to manually click on the “Sign in” button to login – pls. see attached screen capture:
It is also possible to embed the password in the URL itself (e.g. https://<user>:<password>@localhost:8384). I find this useful when I want to have Syncthing protected from remote access but also be able to access it easily locally.
Please check http://kb.mozillazine.org/Network.http.phishy-userpass-length. This is a hidden preference that needs to be added to about:config manually. Also, according to the description, there are some security-related implications of disabling the confirmation dialog entirely, although I’m not sure how relevant it really is, seeing that other browsers don’t use such methods to block automated logins.