- Install Its' All Text! add-on https://addons.mozilla.org/ja/firefox/addon/4125 on Firefox.
- Configure an external editor for It's All Text!. Emacs or emacsclient as you would like.
- Visit Evernote mobile web http://www.evernote.com/m with Firefox and open the note that you are to edit.
- You will see "Edit" icon aside the textbox. Click it to to open the note with the external editor you specified.
- Note: This procedure does not work for the PC page http://www.evernote.com because they use a JavaScript editor instead of textarea there.