"Signing cannot be verified"

I have been unable to install new updates of Foxmarks for some time (using Mac OS 10.4.11) and cannot upgrade to Xmarks. Terribly frustrating, as it has been a precious tool for switching between Windows and Mac OS on an intel computer. The ultimate error after stating that Foxmarks cannot be installed because "signing could not be verified". I've allowed all certificates on Firefox (3.0.5), cookies, etc. to now avail. Any ideas?