The window is supposed to be resizable. The fact that you cannot resize it would indicate that one or more errors are occurring somewhere during the process.
Look for PEP related errors in the error console (
Tools > Web Developer > Error Console) Please install the
Console² extension for error checking. It is more comprehensive.
Try this: Open and clear the Error console. Leave it open and go through the motions of adding a new pastetext item. Examine the error console for any
PEP related error messages (type
pasteemail in the "Search:" box). Copy/paste them here.
If no errors are present, let me know and I will explore the possibility that the problem lies within Fx 27b2 and/or the nl (Dutch) locale. You are using Dutch, correct?