Case #1: the Monster vs. Aliens movie website. Since this has a big glitzy Flash movie, it wanted the whole screen.
Case #2: the Adobe Downloader applet. This cute little Java thingy doesn't *need* much space, so it resizes the window to be Eeentsy-Weeentsy.
What do they have in common? Well, since I work in tab-based Firefox, they both resize MY MAIN BROWSER WINDOW. Never mind that the new page is only one of many tabs I have in that window, it wants to change the window to be the size *it* wants.
Bloody stupid anti-social webpages. Time for everybody to get the clue that we're living in a tab-based world now, and if you're not super-careful to make sure this is a separate popup window, you're going to just annoy a fair fraction of your users.
(Yes, there is probably a Firefox setting somewhere to prevent this from happening. There's a Firefox setting for *everything*. But I shouldn't have to go hunting for such a thing...)