Well it looks like the add-on is dead. Not officially, but it's stopped working and unless I can find a way to get an old copy of Firefox and prevent upgrades, I'm locked out. Too bad, it was a nice addition.
Microstock Monitor 1.0 could not be installed because it is not compatible with Firefox 3.0.3
That's the message I get from Firefox.
After following the link and reading here, it looks like this has been the case for some time.