Create customized buttons on shortcut menu of Firefox and Opera

I'm not sure which newsgroup this question should go to since it involves
elements of all.

A few years ago I created a bunch of custom buttons on my IE 6 shortcut menu
that did special tasks. I created the buttons by going to Registry at this
address:

HKEY_CURRENT_USER\Software\Microsoft\Internet Explorer\MenuExt

and created new subkeys. (I forget the correct term so I'm calling them
subkeys.). In each subkey I would insert the address of a particular html
file on my computer. In the source code of each html file was script I had
written that would run specified tasks on the active web page when I clicked
the specified button on the short cut menu. For example, if I have a web
page open, and have selected some text on that page, and click a certain
custom button on the shortcut menu, it runs script which opens a new Word
document and inserts into it the url and title of that webpage and the
selected text, all in one step.

My question is: is it possible to do the same kind of customization with
Firefox and Opera? That is, add a button to the shorcut menu (right-click
menu) which will run the script in the source file of a specified html file
on my computer?

Thanks,
Larry