Nice ...
However, for (my) real convenience, I would modify it like this :
a) limit the writing zone to the sidebar ... so to have the web page on the right without switching tabs ... and without using notepad++ and switching from a webpage to notepad++ with CTRL+tab
b) add a shortcut such as F4 to open/close it quickly in the sidebar ...

There is no use to replace NOTEPAD++ within Firefox ... the only real use of such an addon is giving user a chance to write or copy and paste in the sidebar links, snippets of text, etc ...

;-)