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?
But Firefox is an ad supported browser and has been for nearly 2 decades.
That they want to take ownership of the advertising is no surprise. Who knows when google will turn off the faucet.
But this is definitely not the right way