Some applications open a popup to fill in some information, when you are using firefox this not always goes the way you want, making is impossible, or nearly impossible to fill in the data, especially if you are not able to resize the window.

To make sure you can ALWAYS resize the windows you can do the following:

Start firefox and type in the URL bar: about:config
This will open the configuration window, now type there:

And set the value to: true, and you are ready to go!