For me, on Firefox 2, the problem seems to be some combination of the Skype plugin, the RealPlayer Browser Record plugin, and being signed into my Google account.  I had already uninstalled the Skype plugin and had intermittent success in the past.  When having problems today, I did some combination like disabling the Record plugin, signing out of Google, restarting the browser, enabling the plugin, restarting, and signing back in. 

That seems to have it working for the moment, but it seems to come and go.  Clearing the cache didn't seem to have any effect in the past, but I'll try to notice the next time I use the Record plugin if it makes Maps stop working afterwards.

Mary