I happen to love Extensions in Firefox, they add so many great features. However I’ve never felt any inclination to add extensions to Safari. That inclination was proved sound a few days ago, when a client in Shepton Mallet has a problem with their Mac Safari. For some reason the Toolbar kept disappearing, leaving here unable to search Google or even type an address into the address bar. You could even get the customise toolbar menu to work.
Nuking Preferences temporarily solved it, but shortly after it came back. It was only after I noticed that she had Tab-Expose installed that the penny dropped. After removing it everything behaved as expected.
So the lesson is, when Safari is playing silly buggers, disable Add-ons.