How embarrassing!
When checking the Application defaults for Firefox I failed to notice that there are TWO PDF entries:
PDF Document -- which was set to Use (default)
Portable Document Format (PDF) -- which was set to Save File

Why are there two entries?

Anyway it's all working now I have set the second entry to Use Document Viewer (default) 
 
-----Original message-----
> From:Bill Maidment <[log in to unmask]>
> Sent: Wednesday 1st February 2017 12:51
> To: Bill Maidment <[log in to unmask]>; [log in to unmask] <[log in to unmask]>
> Subject: RE: PHP issue with Firefox-45.7.0-1.el7_3.x86_64
> 
> Further information:
> 
> The PHP script creates a PDF temporary file (which succeeds) and passes that file to the browser to display it as follows.
> 
> header('Content-Type: application/pdf');
> readfile($sTempFile);
> 
> But all I get is a Save popup for the PHP script.
> 
>  
>  
> -----Original message-----
> > From:Bill Maidment <[log in to unmask]>
> > Sent: Wednesday 1st February 2017 12:38
> > To: [log in to unmask] <[log in to unmask]>
> > Subject: PHP issue with Firefox-45.7.0-1.el7_3.x86_64
> > 
> > Hi gang
> > I am having trouble running an onclick PHP script on an internal web site using the latest Firefox browser. The site was running OK prior to upgrading to 7.3.
> > I see lots of errors in the Browser Console like:
> > NS_ERROR_FAILURE: Component returned failure code: 0x80004005 (NS_ERROR_FAILURE) [nsIURI.host]
> > and pointing to contentPolicy.js line 182 (see attached image of that part of the js)
> > 
> > Is this some new security check? Has anyone else seen something similar?
> > 
> > Cheers
> > Bill Maidment
> 
>