Thanks to Mozilla Links for pointing me towards this Firefox extension. It’s All Text sounds like a very neat tool indeed.
It’s All Text! is a Firefox extension that let’s you edit text entries you may need to do in your preferred text editor. It adds as small transparent button to a corner of text areas which you can press to launch your selected text editor with a uniquely named file IAT! creates.
When you want a little more control over your Web text editing, it sounds like this extension might be the way to go.