Deepest Sender is a client that will allow you to post to blogs from directly within Firefox. It is primarily a LiveJournal client, although it supports Blogger (GData) and WordPress (metaWeblog) too.

This is the fourth add-on I am trying. Deepest Sender has one thing on it's toolbar that I kind of thought was important, the "increase indent" and "decrease indent" buttons. ScribeFire only had a "quote" button which is essentially the "increase indent", to take something out of a "quote block" you have to edit the source. Weak. Deepest Sender also supports categories, so here's hoping that the "Post" button actually does something.

Update: Not only did the post button work, but the above portion of the post remains unchanged from what Deepest Sender posted. I think we have a winner!