After just a couple of weeks spent making it, I've released Mimulus. It's an XHTML editor extension for Firefox, and it lets you edit content right inside the tab. It'll bring up a cursor so that you can just click inside a paragraph or heading and start typing; and it enables key combinations to make new elements, such as Ctrl+U to make a new unordered list, which you can then type into.
Though it's pretty lightweight, focussing on paragraphs and headings and lists and links rather than embedded content and tables and style, it has a source edit mode so that you can make more extensive changes almost as quickly if need be. The main advantage it brings is convenience, for when you need to edit lots of very simple notes very quickly. But though I wrote it mainly for creating notes, I've found that it's actually very suitable for editing random existing pages, and typing up documentation and so on.
It basically takes WYSIWYG to the limit, because the finished product is literally what you see in the browser: there are no toolbars or anything like that since all status information is displayed in the browser's status bar. It's minimally invasive too, so it's turned on per-tab rather than being activated for all of your tabs. And since it's an extension, security is handled transparently, and you can save edited documents to your local hard drive.
And of course it's an open source present, released under a BSD style license. The hard work of doing the conversion to an extension was done by Christopher Schmidt, who also helped out with a bit of the coding and hosts the subversion repository for the code; and the logos were designed by Cody Woodard, who just so happens to be awesome at that.
Please feel free to send me feedback on Mimulus, especially about bugs and so on. Please remember, if mailing feature requests, that the lightweight and minimally invasive characterists of Mimulus are by design and should carry through to any new capabilities.