Can a Firefox/Thunderbird Extension modify a printed page?

Question

I'm writing a Thunderbird extension that performs a translation of the current message on demand. The translation is appended to the HTML in the message view pane (so it is not persistent).

Can a Thunderbird (or Firefox) extension modify the content sent to a printer? I'd like my users to be able to print the translations alongside the message.

Note: This question is meant to focus on "is it possible" and not on how I designed the extension (since ATM its just a PoC).

Answer 1

SmartPrint

This software program will eliminate paper waste by analyzing documents as they are printed and altering a temporary copy of the document.

So yes, it's possible.