I have noticed this just since yesterday.when using the Google search bar in Firefox then clicking on a link from the results I sometimes get directed to random sites with nothing at all to do with the link or some times to another page of links related to what I want.If I open google normally the searches send me to the link in question.
Is there some different setting that Firefox uses for the searches.
Not that I'm aware of, Baz, it's working fine for me. Try disabling all your addons then re-enabling them one at a time.
Don't forget to test in between.