If you want to log in like this, Firefox will show you a dialog where you have to confirm that you want to login directly into the website.

This is done for some securityreasons because it’s possible, like the dialog says, to trick a user to the wrong site.
Take for instance the following url:

http://www.eek.be&login:password@example.com/badsite

This url looks perfectly valid but if you have a good look at it, you will see that the url is not one of eek.be but one of example.com. So when a user clicks this link, it will be redirected to a page he didn’t expected or worse, a site that looks like the page he thinks he expects which asks him for his creditcard/username/password/…

So this built in security in Firefox is a very good case.
But in some cases, you just want to login to a site without that annoying alertbox. Luckily, In firefox, it’s possible to configure this option.
Go to the options in Firefox by surfing to:

about:config

You should get a warning but you can proceed without any concerns.

Now you have to rightclick inside the optionsscreen and choose new->integer
A popup shows up and you have to fill in the optionsname

network.http.phishy-userpass-length

secondly, you have to give a value to the option.
In this case, the value means how many characters a login can have before the dialog appears. By default it’s 1 in Firefox. In this case I change it to 100 which is enough in most cases.
If you want to disable it always, you have to set the integer to 255.