Opening files with Portable Firefox

First of all thank you very much for this great application.
Now I use PortableFirefox from HDD as standard windows application and it's pretty nice way to keep everything what I need in one easy-to-copy directory.
The only thing which I couldn't realize is how can I open some html or even mht (I installed the appropriate extension) file with PFF doing this with right-click mouse menu and Open with... option. The default browser is still Internet Explorer. Is it possible to have some command line parameters including filename upon PortableFirefox.exe startup?

Thanks in advance.

AK