Writing is not required in this case. The contract won't take more than a year to perform. Of course accepting in the same way you received the offer is acceptable (section 65), but so is any reasonable method (section 30), unless of course they specify a particular method (section 60).
See, this is the problem with some people--you're probably a good STUDENT who's able to state the law verbatim, but you need to take off your 1L cap and think practically if you're ever going to succeed as a LAWYER. A lot of people are never able to make that transition and end up sucking in the real world (or they become lifetime academics).
You need to think BEYOND the MINIMUM requirements and look at the big picture. Yes, technically, a law firm COULD slide by with an oral acceptance and some argument in court if it came down to it. BUT, do you think a firm, with vast legal knowledge of the consequences, would really want to leave the door open to any question of whether you accepted? Of course not -- this is precisely what separates GOOD lawyers from TTT lawyers. A TTT lawyer would tell you that allowing oral acceptants is fine (as you've done here), but a GOOD lawyer would tell you to make sure the motherf'er puts it in ironclad, unambiguous writing. So as far as I'm concerned, yes, you MUST put your acceptance in writing of some type no matter what the law says--and if the law firm doesn't require it, then they're TTT and you should work somewhere else. And PS: email DOES count as a "writing"--I did not say it had to be mailed.