It does seem that QA is generally better in Debian, but I've all but given up because the development lifecycle in Debian is far too long to cope with how often major software applications release updates these days.
Which is why there is always the backports repository for stable, and more desktop-oriented uses can always use testing. I find testing to be nearly as up-to-date as Ubuntu, and more stable.
For example, until now the "stable" version of Debian still contained Firefox 2.x.x.x
People who give examples like this are missing the point. RHEL is the same way. You realize RHEL was on Firefox 1.5 until the 5.2 release half a year ago? Again, on a server you want stability and not rolling upgrades. Who cares about browser and codec versions on a server?
The number of developers contributing to Ubuntu now is orders of magnitude higher than in Debian, so it's completely possible that things get fixed much quicker in Ubuntu these days.
Uh, no. Orders of magnitude higher? You realize Debian releases for 11 official architectures? Ubuntu only releases for i386 and amd64 now (and before that they only had ppc and sparc in addition). And ~80% of Ubuntu's packages still come from Debian unmodified. Debian has > 2000 active developers maintaining packages. Does Ubuntu have 20k developers? According to launchpad, Ubuntu has 135 MOTU members, 106 contributing universe devels and 72 core technical board members (including 56 inactive). They can respond faster because their process and organization is different, but also because they have a lot less work to do since they leverage the work of Debian developers. They only have to focus a vastly smaller subset of packages on a few mainstream architectures. The rest is done for them, courtesy of Debian.