I have been looking for an easier way to edit blogs and Wiki with a text editor. I use this mainly for the spell checking aspects. The Mozex
plugin works with Firefox and allows you to specify an external editor to edit text boxes. When you save the document, it puts the contents into the text box. You can configure a "hot-key" to start editing or simply do a right mouse click.
On OSX, you have to use the "open" command in-order to run Aqua apps using this plugin. This is configured in the Tools->Extensions->Preferences.
/usr/bin/open /Applications/TextEdit.app %t
Newer versions of Firefox include an inline spell checker. I have tested it on 1.5 on OSX and 2.0beta on Solaris.