Website Developer Question:

There is a right answer to this question: all of them. A competent developer should be familiar with testing cross-browser compatibility by using all the major web browsers. Obviously they'll have a primary browser they use for surfing, but their answer to this question might be a good way for you to segue to asking how extensively they test cross-browser issues. Also, if it's some kind of css/html position seeing what toolbars they have installed can be a good metric of their skillset.