Apparently Firefox will only use the integrated PDF Viewer if the server sends the file as 'application/pdf'.
See cor-el's 3rd post in ==>>
Why can't i open PDF files...?
At the bottom of this post
[GUI Plugin] QuarantineAndScrub there's a pdf attachment, I can't open it in the built in Firefox reader
But I can open this one at the calibre site (in the last line of intro) ==>>
calibre User Manual
Conclusion - Mobileread is not sending the PDF link as 'application/PDF'
Can the server can be adjusted so that links to PDF's can be read in with Firefox's viewer?
BR