The DEVONthink menu in the services menu is all grayed out, so I can’t use any key combination to take a rich (or plain text) note. Is there a way around this?
No. I don’t think Firefox is compatible with Mac OS X Services. Earlier versions of Firefox were not Services-compliant; I haven’t looked at the latest version.
The bookmarklets included in the download disk image, inside the Extras folder, will allow capture of bookmark files (URLs) or WebArchive files from Firefox to a DT Pro or DT Pro Office database. But that’s not a keyboard operation.