To be fair, what I have heard (I am not a Mozilla insider so I can't say personally) is that donations go to the Mozilla Foundation and the commercial agreements give money to Mozilla Corporation. So you aren't donating to the same place.

From the same source, apparently the Mozilla Foundation does a lot of important work that the Mozilla Corporation can't/won't do but most of the development is done by the Mozilla Corporation currently (possibly due to not enough donations to the Foundation?).

I haven't really verified that, but thought it was worth pointing out.