It's All Text
It's all text is a great tool for getting firefox to co-operate with an editor program to edit WW documents.
It's all text should open documents from the firefox editor the same way your computer opens text documents by default. On a Mac, you will probably want to control It's all text this way. On a Windows or Linux machine you can also manually set the editor in your IAT preferences, if you prefer.
You can preview wiki documents and still keep the link between IAT and firefox, but there is a small glitch in this process. The first time MW opens a page for editing, the end of the URL says "action=edit", whereas after subsequent previews, it says "action=submit". This change breaks the link between the edit window and the browser window.
You can address this by always previewing before you edit, or by creating a monobook.js page. For now, this has to be done for each wiki, which is a bit of a pain, but can be done pretty quickly.