I have used IE (9 - 11), FireFox and Chrome. IE has problems displaying the Web Viewer. FireFox won't put focus on an input field when you tab between fields (you have to click on each input field to enter data). Chrome seems to be the best solution (at least for my application). Unfortunately, my government clients don't seem to trust/like Chrome.
Hope that helps.
FireFox is not recommended by FileMaker for use with Web Direct.