There's a reason that the URL is causing problems. It used to work in all browsers, all the time. Then phishing scammers realized that they could build a page protected by a username and password like this:
user: www.citibank.com
pass: secure
So then the quick way to log into the page looked like this:
http://www.citibank.com:secure@a7d94mf.com/login.php
See, if you look at that URL quickly (or you're a newbie), you will think that you're at the real live citibank page.
So to protect people against that, the helpful people at Firefox put in a confirmation box that says, "You are about to log into a7d94mf.com. Are you sure you want to do that?" I assumed that IE did that too... but instead, from what I gather from your post, it sounds like they just made the URL invalid... which breaks lots of web pages out there that already exist and use that type of URL. Therefore, Microsoft=dorks, Firefox=smart
As for your best solution, I'd drop the .htaccess protection and build a simple PHP based login instead. HTACCESS is extremely secure and a PHP based solution is as secure as you can make it. Take your pick.