Rated 5 out of 5 stars
If you are like me and only want to be able to "Open in External Application" rather than switch rendering engine, you can hack the XPI file to make the max version 3.7 and then install the extension. Opening in IE external window works. Switching rendering engines does not.
Download the XPI file here:
Open the file in Winrar, extract the install.rdf file, open that file in notepad. Search for maxversion, and change it to 3.7. Save file, put it back into the XPI file still open in Winrar. close winrar. Drag xpi file into FF and it will install. Its a temporary solution until the developer updates the add on..