Firefox hijacks Url opening from IE7 (!?!?!?!)

Firefox 1.5.06 is my default browser. It's acting a little strangely with some of the admin facilities on this site, though, so I just opened IE (7, beta 3). OK, but now I discover that when I type a URI into the IE addressbar, a new tab opens in Firefox and loads the page! I mean, there's default and then there's default.