Thunderbird and Firefox are not GTK apps. Also, there are really good Qt apps out there which blow everything else out of the water, such as Amarok (especially the older versions), K3b and Anki.I find it curious that GTK, being the supposedly lesser toolkit, seems to foster great apps such as Chromium, Inkscape, Shotwell, Firefox, Thunderbird, The Gimp etc whereas the Qt side of things have a tendency to get stuck at infrastructure plumbing and overly complex ideas about UIs and outside of KDE, rarely deliver brilliant apps.
The problem with Qt apps is not in the toolkit -- it is professional and probably the best toolkit on the market, for any platform. There are two problems -- the traditional resistance of the Linux community towards it, fueled by the early license issues and by its corporate backing. Big Linux names such as Red Hat chose to throw their weight (and money) behind GTK, which was seen as more community-driven, and to try to bring it up to par. Big funders of Linux felt uncomfortable basing a desktop on a toolkit whose success was largely influenced by a single company, and was therefore difficult to control.
The second problem is that GNOME and GTK apps are heavily funded by Red Hat and co., while KDE is almost exclusively a community effort. In essence, GNOME has always been the corporate desktop (Novell, Red Hat, Sun Microsystems, Ximian, Eazel, Canonical, and who knows how many other corporate backers), while KDE was the community desktop which built upon the corporate-developed Qt.
Personally, I use KDE 4.4 on Debian stable for daily work. It hasn't crashed in over a year. Flamers will flame, and most of the time they have no clue, so you should just find what works for you, and use that. Having used KDE since 1.1.2 on Red Hat 5.3, I find that I like the project and the vision they have. And the undeniable mistakes that they have made pale in comparison (IMHO) with the disastrous decisions behind GNOME. I've experimented with FVWM, Afterstep, WindowMaker, Compiz and Enlightenment, and always end up going back to KDE in the end because it lets me do things the way I want to do them and gives me all the tools I need. Other people should use what best suits their workflow -- for me, GNOME Shell and Unity are simply unusable, even more than a Mac.