I confirm something is not working properly with ShareMeNot and Firefox 20.0.
The addon is up-to-date but when I go to the preferences for it, I notice that all "Block options" have been unselected!
I reselect them back, quit Firefox and restart and everything seems to be working fine. ShareMeNot seems to be working.
However, this is the second time I do this procedure. This means that at a certain point, the "Block options" of the add-on get unselected again and the addon stops blocking everything!