I’ve just come across Deepest Sender, which installs in Firefox as a plugin and connects directly to your blog. It’s a simple install, followed by a wizard to set up the first account.
Once installed, it opens with a pop-up editor window that includes normal, source and preview modes for the post. Categories are available as a dropdown. Posts can be saved as drafts. Tags don’t seem to be an option.
Trying it out now on this post.
It’s available at deepestsender.mozdev.org