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?
For me it's been downhill since they removed "Compact" UI density, and I'd just as soon not jump through a bunch of custom CSS hoops to have sidebar tabs when nearly all the other browsers (outside of Chrome/Safari) are building them in natively. The main thing going for Firefox is being the independent rendering engine, for customization and power user features it's nothing special anymore.