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).
There are some custom fields in Page Setup where you can set a field for the print header or footer, or both. Your best bet would be to populate those fields when printing. I am not sure if there is a hook for that, but you can always replace the Print menu and make your own.
If that does not feel appropriate, you can modify the contents of the page before printing and revert changes after that (like AdBlock or Nuke Anything plugins do).
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.