When I select one of my bookmarks or search (Google) the tab opens to a Yahoo search engine page.
I've uninstalled 'My Search' but when I attempt to reset the settings by typing 'about.config in the address bar I also get the Yahoo search page.
The version of Firefox is 23.0.1 (apparently up to date)
Maybe that's a typo: the internal Firefox pages uses a colon, e.g., about:config, about:newtab, about:about (not joking).
In Firefox 23, the address bar uses the search provider you set on your search bar (the bar with the icon on the left and the magnifying glass on the right). Can you check that it is set to Google?
Bingo! Thanks Jscher2000.