Note you used supportingNote that I used "go out of their way to support". The active role was clearly implied, and consistent with the context.
ZeroGravitas described Apple and Microsoft's choice (to use h.264) as an "anti-competitive action", and in contrasting that choice with an alternative, I thought it was obvious that I meant choosing (acting) differently, i.e. using Theora, in spite of it not being in their own interests. What else might I have meant?
If we were talking about a passive role of "supporting", then Apple (and possibly Microsoft) would have to be excluded, as their <video> implementation (at least on the desktop) allows for installing additional third-party codecs, such as Theora. In that sense Apple supports Theora, but clearly that is not what we're talking about.
I know of no other sense in this context that Apple or Microsoft could support Mozilla without also aiding them.
(The reason I stated this initially in terms of "going out of their way" was to distinguish this situation from situations in which self-interested competitors benefit each other directly, such as how Apple's work on WebKit benefited Google's work on Chrome (and vice versa). There was no argument made to suggest that choosing Theora would have been in the interests of Microsoft or Apple, though, so this was (I thought) redundant, hence the change.)