This situation is much more common than you might think and has been discussed on this forum a number of times.
The first firm will not want you to join them unless you are absolutely sold on joining them. The last thing they need is to force someone to join them who will then mope about and not have the energy levels or enthusiasm needed to serve their clients - and who will drag down morale in the office. I'm not a legal expert, but the worst that I could see happening is that they force you to join them and then you have to work out your notice period (which would be terrible for morale in the firm so I can't imagine any firm actually doing this).
You need to be decided in your mind whether you want to give firm 1 an opportunity to improve their offer - or whether you would still join the new firm 2 even if firm 1 were to match or better this new offer.
Then just call and steer them accordingly. If you decide on the former then you should be clear about this when you call - "your whole attitude to negotiating my salary and giving me a take it or leave it offer has given me serious cause for concern about my remuneration prospects with your firm. Having been through the process with another top firm I've had a totally different experience with them and you've just left me with the impression that my career will not be best-served joining your company."
Might be worth you paying for a 15 minute consultation with a lawyer on this, just to be sure of the ramifications - though I'd have thought some Google searching on the subject would quickly turn up the liability you might face in such a scenario.
Good luck, Tony Restell