What version of Map Rank GL do you have?
What version of Firefox are you using? I am using 8.0 at work and 3.6.something at home and it works both places.
What version of Greasemonkey are you using? What are all the Greasemonkey scripts you are using?
Do you have other Firefox add-ons?
Generally speaking, you need to think about when Map Rank last worked for you, and what has changed since then.