If you can't leave and you really want that NEW job. 2 options
a) Leave OLD firm, get penalised 1 month.. and work for NEW job. Since in your post you said that the NEW firm is better than the old one.

b) Leave OLD firm, get penalised 1 month.. work for NEW job, BUT suggest to the NEW employer whether they can reimburse you for you leaving the OLD firm. (This scenario happened to some of my ex-colleagues, whereby the new firm just paid the penalty and take them in straight away 'cos they were in demand and needed staffs straight away)

What ever you do, do not leave your OLD company on bad terms as it might haunt you in the future. Besides, if you are seeking a testimonial from them when you leave.. more reasons why you should try to find an amicable and diplomatic solution to this.

The world is small, bad judgment on handling this might have repercussions in the future.