Firefox 7 was recently released and has a few behavior changes. One of the changes is designed from preventing sites from resizing the browser window if they didn't create the window object (see Bug 565541 - Web sites shouldn't be allowed to resize main window and What’s new for Web Developers in Firefox 7.)
However, in my testing it appears the window.resizeTo() is now completely broken. I'm not seeing any behavior that allows a window to resize itself. The change in behavior, from what I can determine, should still allow a window to be resized if (a) Can't resize a window/tab that hasn't been created by window.open (b) Can't resize a tab if the tab is in a window with more than one tab. Maybe I'm reading things wrong, but if you open a window via window.open() you should still be able to resize that window.
UPDATE: After some more testing, it appears that Firebug v1.8.3 is the root cause. After creating a clean profile under Firefox 7, the example below worked fine under the clean profile. I then installed Firebug 1.8.3 and the example breaks. Disabling or uninstalling Firebug makes the example start working again.
If you run the following example in any browser but Firefox 7, you should see a pop-up window appear and then it should resize itself twice (in 2 second intervals.) The first resize is from the parent window object (this page) and the second resize is from within the pop-up window itself.
In my testing, this works properly in all browsers, but Firefox 7. Click the link below to see the problem in action.
Open Window