I am horrified by the number of developers who only test in Chrome. At my last gig I found glaring UI bugs (whole menus not responding to mouse clicks, for ex) in FF. Sad.
At my job I work in Firefox, a coworker works in Chrome, and a third person works in Safari (to be honest we don't bother checking the site in any Microsoft browser). That way we are able to do cross browser testing fairly easily.