The closest they say is:

PowerFox is both standards compliant and feature-rich, supporting the latest web technologies to ensure compatibility with modern websites.

which means.. next to nothing. No "PowerFox version X has (at least) the same web standards support as Firefox Y".

it likely has code from newer versions spliced in

That's a good thing, but making it clearer what to expect will make it easier for potential users to decide whether to give it a try.