If you are experiencing issues opening a PDF document in iinsight from a Firefox web browser, it may be your permissions blocking the file from opening in Firefox.

To resolve this problem, In the URL address bar there is usually a Padlock icon, click the Padlock icon then 'More information' (marked with arrows in the below screenshot)

Then chose 'Permissions' and change adobe reader to either ‘Use Default’, ‘Always Ask’ or 'Allow' so now all attachments will open. You then check 'Do this automatically from now on’ when the file opening dialog box came up.