This bug was originally reported to Debian BTS, please see the following link
for more information:
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=519832

The original reporter said:
[...]

I have noticed with mozplugger that when one has kpdf or okular 
installed and one vists a webpage with a PDF, when either of the said 
apps loads the page-up and page-down keys have no effect.  If however 
the PDF's are downloaded and launched directly, the page-X keys do work 
fine.

[...]

Can you test that the latest release 1.13.3 now fixes the problem.

(In reply to comment #1)
> Can you test that the latest release 1.13.3 now fixes the problem.
> 

Uploaded to unstable, I'll let you know.
Thank you!

Seems still reproducible, please see the last message here:
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=519832#31

I think this is the same issue as
https://www.mozdev.org/bugs/show_bug.cgi?id=22359 

There is a general problem when using certain combinations of browser,
application and window manager with the swallow flag. These range from keyboard
focus issues, windows not being swallowed, up to desktop lock up. In all cases
this is because the application is not expecting to be embedded in a browser or
a window manager not expecting another application reparenting windows it
thought it was managing.

Unfortunately I have been unable to find a general fix / work around that can
be applied to mozplugger to cover all cases. Therefore it is recommended not to
use the swallow flag in these cases. Instead use the links flag or if the
application supports it, explicitly tell the application which window to embed
to.

I have two questions:

1) Previously, the bug was declared as fixed. How is it possible?

2) If the application is not expecting to be embedded in the window (so it
cannot handle input), why does the mouse wheel scrolling work, instead?

I look forward for some answers, thanks.