While I am not sure what the Times New Roman font has to do with professionalism :), I can tell you how to change that.
That font you see in that dialogue box is actually controlled by your browser's default font - which is set to Times New Roman if you have never adjusted it.
Go into your browser preferences and change to whatever you like. Attached is my preferences screen for Firefox - which as you will see has the default font set to Arial with a size of 12 - so that's what font my dialogue boxes use.
Hope this helps,