So, the thoughts:
* I would refuse to testify against my former employer unless I were subpoenaed. Even then, I may move to quash the subpoena (this may be different in your case, of course).
* Being subpoenaed could include depositions and travel and courtroom testimony and rescheduled evidentiary hearings because the judge has a murder trial that's running long -- it could take weeks of your life. This is not an hourlong process. Also, everyone else involved in this process is going to be paid for their time; who pays for yours? Do you even want, ethically speaking, to be paid for your time? Remember the "major law firm" does not work for you and does not have your best interests at heart; their interests may be opposed to yours. To protect yourself you could hire your own attorney, who isn't going to work for free.
* If your former employer has an active fanbase and strongly and convincingly denies your allegations, you could be attacked by name, and your family members dragged into this, online and via social media.
* There are innumerable laws and regulations, some of them rather nuts, and at least one employee at every large company may have innocently violated one of them at some point. Do the laws you believe your ex-employer violated punish behavior that you personally believe should be illegal (malum in se vs. malum prohibitum)? And did management know about and approve these illegal actions, or was it a rogue employee at fault? If the answer to either question is "no," you may want to drop this.
* Now that you've been contacted by a law firm, you could be subpoenaed even if you do not respond and try to drop this.
* If you believe your former employer has institutionally engaged in actions that are illegal and unethical, you have other options beyond testifying. You could write up your experiences and post them publicly, anonymously if necessary.