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?
Introduced by a bunch of gaslighting that it isn't actually happening or isn't anything different that what was always happened, then interleaved with accusations of bullying and entitlement directed at its userbase.