Export/Import text from/into email textbox in Firefox to/from the text file

[olegyusim]

Hi, guys,

I got school project to create an extension for Firefox, which will
allow
user to sign digitally email message, opened in Firefox. I do not have
problems any part of this project, except 2 moments:
1) I do not have any idea how to export text, which I'm going to
encrypt, from Firefox to text file.
2) Same way, I do not have any idea how to import cipher text from text
file to Firefox.
I will appreciate any advice because so far I'm completely lost.
Thanks in advance,

Oleg