There is a known issue with Firefox in that it will not allow you to complete PDF forms in the browser window unles you configure Firefox to use Adobe Reader as the pdf viewer instead of the Firefox built in pdf viewer
You have two options to get around this problem:
Start by going to your personal preferences.
Click the preferences button on the main start screen or on any other screen once you are logged in.
Set the store the documents download option to download all.
All stored documents will now download to your browser's downloads folder instead of opening in the browser.