Stop being law students and start being people. This isn't a contracts issue or an employment at will issue. Its a politics and people issue. Is firm A in your school's area? Do they typically recruit from your school? In other words, would you be throwing your school's rep under the bus by potentially renegging? And if so, does it matter? Is there any way that you could split the baby here?
The first thing you need to do is find out what this means for you vis-a-vis your career services office. If you're breaking NALP rules, you may be cutting yourself off from your school's career services office.
I would probably go on the interview and if things start getting down to brass tacks then you can figure out what to do. Are you a 1L? If so, then you could use this as a get to know you interview and then play this thing off the bounce for next summer. Also, what is the profile of firm 1? If it is just a small firm, I would suspect that they will understand your situation - i.e. that you would prefer something with the potential for future employment. Its not like not having you is going to cause the firm to go under.