I hope he has checked this with a lawyer and that he is not giving his adversary ammunition. Good luck.
At least, for the US-side lawyers (I'm presuming Mozilla's US lawyers are on this one) due to conflict-of-interest rules, they would not be able to step in to offer advice and guidance to him directly, because the representation would probably be adverse to Mozilla's interests.
The rule in most states (based on the ABA model rules) is " A concurrent conflict of interest exists if: (1) the representation of one client will be directly adverse to another client; or (2) there is a significant risk that the representation of one or more clients will be materially limited by the lawyer's responsibilities to another client , a former client or a third person or by a personal interest of the lawyer."
Even if you don't believe #1 applies, #2 definitely applies.
Norway may have different rules, but the local Mozilla lawyers are bound by their local state bars, where this rule (or some variant) would apply. I've avoided discussion of whether this conflict could be consented to by him, etc, as it varies from state state
This of course, only applies directly. They can and probably are talking with his lawyers, just not advising him directly.
Even if they don't have someone handy who knows Norwegian law, they could easily retain them.
My point is that proper legal advice can not be what's preventing them from helping. My question to OP was about the differences in laws which he/she was referring to.
Assuming he can back up his narrative with facts in court, I cannot imagine Opera has a leg to stand on. If he followed up the conversation with the CEO with a clear email, then there is documentation in place. That the CEO hasn't responded should have no bearing, as an oral agreement is considered as binding as a contract in Norway.
Lawyers do not make decisions (or at least, should not). We exist to assess risk, analyze liabilities, advise clients, and hopefully, present options and strategies to minimize risk while still accomplishing a given goal.
Any smart lawyer in this case knows part of him still being hirable is going to be not having his entire reputation destroyed. Depending on the accusations, "say nothing" may not be a necessary or even valid strategy. Additionally, the client is the one paying. If they strongly feel they want to defend themselves, your job as a lawyer is to try to make sure it does not legally jeopardize their case (or warn them if it will and they want to do it anyway).
In any case, note that civil cases in most countries are strongly pushed to settlements. This is one of the reasons for broad discovery in most places.
All of this said, lawsuits against employees like this are pretty darn rare. Having been involved in a number of these situations on the other side, usually they are just bluster, basically a warning that "if he works on something competitive, we will come after you", or a warning shot to stop poaching. An actual lawsuit is usually reserved for exec and above, which i don't think he was (Sorry if i'm misunderstanding the level/ladder distinctions Opera uses :P)
Lawsuits like this tend to only result when someone has really pissed someone else off, or plans on doing something one side sees as really damaging.
Again, the above is my experience in the US, Norway may be different.
Outside of the actual legal realm, there is always a hearts and minds war. People stop paying attention to this stuff very quickly. Whatever gets said in the first few days will be the narrative for a long time. After the first few hacker news posts, this will go away for a year or two, and then someone will win or lose. In the meantime, if he doesn't speak, everyone will think of him as "that guy that stole stuff from Opera". On the other hand, if he frames the narrative right, it becomes "Opera, that company that is going down the tubes because it has resorted to suing former consultants on baseless charges".
Since nobody really wants to go to court, or lose mindshare (In this case, it's very valuable to both sides for very different reasons), whoever frames the narrative best tends to extract the most favorable settlement terms, or at least, go into the suit with a better stance and ability to press claims harder.
In an actual trial of course, it won't make a lick of difference.
He means the obvious "strongly advise him not to do it", in effect stopping him saying anything.
You over-thought that response too much!
Edit: slippery pronoun