Tech support responded by simply sending me a copy of the supported-browser list.
I sent back a response telling them why Firefox should indeed be working with their site and explaining the pedigree of Firefox and reminding them that I'm a Mac user for debugging purposes.
Their response? They told me, a Mac user, to use Internet Explorer! And furthermore, the wording of the second response I got treated me as if I'm an idiot who doesn't know what I'm doing. Apparently, they don't know the systems that they're supporting ... nor do they pay attention to important information in users' reports.
Any suggestions for who else is any good? I like the fact that H&R provides PDF copies of the forms for you to keep handy, even after they e-file for you. I'm obviously not impressed by H&R's attitude or programming team. You don't break what isn't broken and you don't ignore what users are telling you about things that make them unhappy.