At least it was a small scale experiment and not something that rolled out to the whole install base, I use Firefox on a couple of computers and didn't see it myself. But should you really need user feedback to know that inserting an overlay that looks like in-page ad content is a bad idea?
An apology needs three parts: admission that you did wrong, expressing regret for your wrongdoing, and a change in your behavior so that you don't do it again.
Mozilla's tendency to just do the first two and skip the third means that, in my view, those weren't real apologies.