Brill, thanks a lot. I just don't want to do anything unethical. FWIW firm 1 gave me under a week to respond...They have offered you a job. Clearly they don’t care about you reneging another firm’s offer. They may just might be concerned you do the same to them, but clearly that’s a risk they are willing to take.
Reneges happen a lot in recruitment (less in law for TCs due to the time lag and financial commitments) - recruiters don’t really care if you renege. It’s better you renege now than train and leave.