Is it acceptable to ask how a professor would like to be addressed through email?
In contrast to other answers voiced here, to me, signing a mail with {Firstname} is a clear indication that one can address the sender by {Firstname}. It would be unreasonable for the prof signing with {Firstname} to expect otherwise. I don't agree with the "always better to play safe"-advice, as I strongly believe in that an academic discussion should be based on arguments, not on status. If I expected my students to call me in a formal way, then I would have to address them in a more formal way as well (at least, that's what I feel). When I indicate that I as prof feel comfortable with the informal route (which I do by signing my mails in this way), there is no reason not to accept that offer.
Having said that, it is of course highly dependent on culture and language.