I had this problem as well (believe it's a problem specifically with Firefox) and found an article on how to fix it, but lost the link and am in the same boat again.
I vaguely remember that if you go to Options > Content > Javacript > Advanced and make sure there isn't a check mark next to "Resize windows" that should solve it.
If that doesn't work, you can do a hack that allows you to manually resize the window (always) if it's too small. Type "about:config" (without quotes) in the address bar and type "dom.disable_window_open_feature.resizable", double click on it so it switches to true.
Hope that helps someone out :)