Here's a more complete instruction, based on fero14041's post.
Download document_map-0.6.1-fx.xpi from Mozilla, but use Internet Explorer instead of FireFox. Rename file to .zip, then unzip it into a directory. Edit "install.rdf" in the unzipped directory and change the value of "em:maxVersion" to "3.0.1" from the old value of "3.0a1". Create a new zip file of the (modified) directory, and rename the .zip file to .xpi. Drag and drop the new .xpi file into FireFox.
This great extension seems to work with FF 3.0.1. As Brett Zamir said, you have to modify file "install.rdf" in extension archive .xpi (it's a zip file), and change value of "em:maxVersion" with "3.0.1" instead of "3.0a1". Enjoy...
I think this is a great addon, especially when looking at online documents (like IETF RFCs). I'd like to echo the other user requests, though - the expansion of the doc contents in the bar on the left should be controllable. You should be able to right-click and select a level of detail (or at the very least, say expand all or collapse all). it's quite bothersome to have to manually collapse 25 sections every time I come back to the same page.