If you mean "legal" and "illegal" conduct, as defining a crime, the answer is probably, but prosecution by federal of local goverment is doubtful, unless you are decompiling code belonging to a company with powerful connections.

If you mean "illegal" in the sense of a judgment arising from a civil lawsuit, the answer is "hell, yes".

The Refox license agreement doesn't apply to decompiling an application that belongs to someone else. The Refox license is between you and Refox and cannot govern the rights of third parties.

The real question that is implied in your post is that someone might consider decompiling for the purpose or with the intent to use somebody else's solution; which doesn't necessarily mean an outright cut and paste of another's code. You could just be seeking a business advantage by looking through someone else's intellectual property for marketing or for reverse engineering.

Unless the EULA expressly allows redistribution, modification of the source and/or declares the code to be in the public domain, and assuming that it can be proved that you "did it", it's a pretty easy lawsuit for the plaintiff to win.

If you are looking for a specific solution, there are a ton of public domain sources in the Fox community, including this site.

Regards
- Don Lowrey