Edit in TextMate (from Firefox)
While I have mentioned this before, TextMate comes with an input manager you can install. Hereafter you can press ⌃⌘E in any Cocoa text field (even editable WebViews) to send the text to TextMate, where saving and then closing the window will take you back to the calling application (with the updated text).
Recently Brett Terpstra wrote about how to get similar functionality working for Firefox (which is not using Cocoa text fields).