So interestingly, since the whole “bad sound quality” debacle quieted down, Bose have removed this post from their site (it redirects to an admin login screen now) but you can still use this trick to see all advertised firmwares if you want/need it https://web.archive.org/web/20170129012654/https://community.bose.com/t5/Wireless-Headphones/Audio-Quality-Concerns-Since-Bose-Connect-QC35-1-2-9-Update/td-p/35518
(I don’t think the “bad sound quality” was ever Bose’s fault; it sounds like it was people with shitty phones and computers putting the thing in bad bluetooth protocols, but hey, nothing like a bit of internet controversy)