For somebody who doesn't want to talk about the case, he sure does talk about the case an awful lot!
With that said, I think it's a very good strategy for Trond to go public with this. He is one person and the other party is a corporation. It will be a lopsided fight in the court. Even if he would win in court, it will have cost him dearly. This is what these kinds of suit about - intimidation. But by going public, he will be able to get public support and Opera is risking eroding their brand and good will. At the end of the day, a cold calculation on Opera's part to see how much branding they are losing due to the suit will convince them to drop it.
edit: Correction by magclock. Mozilla (so far) isn't directly involved in the suit. https://news.ycombinator.com/item?id=5629534
It'll be interesting to see how this plays out.
It's pretty typical that what happens is the corporation gives some initial advice, and tries to find you good counsel. They basically have to get out, because the corporate employment lawyer does not work for the employee, and there is usually a clear conflict of interest between representing both employee and corporation interests at the same time.
Since Mozilla was not sued here, Mozilla may pay for his defense (depending on factors), but direct intervention usually does not happen.
The rest goes to your "public opinion" element. Mozilla may publicly support him, of course, but in most cases, associating yourself with a lawsuit is often lose lose, even in cases where it appears to be defending a good guy (Imagine if today they support him, and tomorrow it turns out he was lying).
They may just be hoping their name doesn't get dragged through the mud here. I haven't seen any public release from Opera accusing Mozilla of bad behavior, but if Mozilla comes out and says something very supportive, Opera may just do that. My guess is staying out of it publicly seems the best way to accomplish that.