IE7 bug?

I've started trying out the recently-released Internet Explorer 7 Beta 2 and it's a big improvement over IE6. There's been plenty mentioned about CSS bugs that are either still there from IE6 or new to IE7, but I just came across a really weird one that might just be a setting somewhere, but until someone shows me how to change it, I'm assuming it's a bug. I've been working on a layout for a new website today and have just been sticking to Firefox for testing as I put the basic page together. With that done, I went to open the file in IE7 and it seemed to be refreshing the page in the Firefox window on the other monitor. After a few more attempts to open the file in IE7 (I'm slow, I know), it dawned on me that it was actually loading the file in the default apllication for the filetype as it's a local file, not one from the web. As Firefox is my default browser and it's an HTML file, IE7 was kindly opening it in Firefox for me. There are probably instances where this is helpful, but trying to test compatibility of some CSS in IE when IE isn't your default browser is certainly not one of them. On the assumption this is a bug and not an option, I'm posting it over on the Microsoft discussion group to see what's going on...