Can I be fired for an on-linecomment that I made last night off the clock?

It depends. As a general rule, you can be terminated for this. In an at will employment relationship an employer can pretty much set the terms and conditions of your employment as it deems fit. This includes comments made on social networking sites. So typically, unless you have an employment contract or union agreement that prohibits your discharge, or your dismissal violated existing company policy, or your treatment constituted some form of actionable discrimination, you have no legal recourse here.

However, there is an exception regarding the firing of an employee for making negative on-line comments about their employer. If the griping amounted to "protected concerted activity", an employer cannot fire a worker under federal labor law. In some instances, the Facebook chatter may be no different from workers gathering around the water cooler to discuss working conditions. The National Labor Relations Act prohibits employers from punishing employees, whether or not they are members of a union, for talking about wages or workplace conditions or forming a union. The idea is to more easily enable communication among workers so that they can decide if a union is necessary.

So whether or not you can be fired depends on the circumstances surrounding your comments. If it was just for negative remarks made to your family and friends with no other purpose, then yes you may lose your job over this. If it was to made to other employees for the purpose of improving working conditions, then you are probably protected. 

At this point, if you still have questions you should consult with an employment law attorney.