If you follow this guide, you should be able to get PDF files to open inline, embedded within the Firefox window as in the screenshot below:

To allow PDF files to be opened inline within the browser window, you need to do the following:

Install Mozplugger and Evince:

sudo apt-get install evince mozplugger

Edit the Mozplugger config file:

gksudo gedit /etc/mozpluggerrc

Find the following block of text (or something very similar):

application/pdf:pdf:PDF file
application/x-pdf:pdf:PDF file
text/pdf:pdf:PDF file
text/x-pdf:pdf:PDF file
        ACROREAD()
        repeat noisy swallow(kpdf) fill: kpdf "$file"
        repeat noisy swallow(Xpdf) fill: xpdf -g +9000+9000 "$file"
        repeat noisy swallow(okular) fill: okular "$file"
        GV()
        repeat noisy fill exits: evince "$file"

If you prefer to use the Evince PDF viewer replace the above code with the following:

application/pdf:pdf:PDF file
application/x-pdf:pdf:PDF file
text/pdf:pdf:PDF file
text/x-pdf:pdf:PDF file
	repeat noisy swallow(evince) fill: evince "$file"
	ACROREAD()
	repeat noisy swallow(kpdf) fill: kpdf "$file"
	repeat noisy swallow(Xpdf) fill: xpdf -g +9000+9000 "$file"
	repeat noisy swallow(okular) fill: okular "$file"
	GV()
	repeat noisy fill exits: evince "$file"

Or if you prefer to use the xpdf PDF viewer, you can use the following lines (make sure you if not already installed that you install xpdf using the command sudo apt-get install xpdf):

application/pdf:pdf:PDF file
application/x-pdf:pdf:PDF file
text/pdf:pdf:PDF file
text/x-pdf:pdf:PDF file
	repeat noisy swallow(Xpdf) fill: xpdf "$file"
	ACROREAD()
	repeat noisy swallow(kpdf) fill: kpdf "$file"
	repeat noisy swallow(okular) fill: okular "$file"
	GV()
	repeat noisy fill exits: xpdf "$file"

Save changes, close all instances of Firefox and Evince/Xpdf and then open firefox and try viewing a PDF file. Everything should now function as you want it to.