This was one of the more easy errors to fix - that is _after_ I realized what the problem was, because the error message didn't help at all...
So if you get something like:
mozilla-central/widget/src/qt/nsWindow.cpp:259: error: 'OpenGL2' is not a member of 'QPaintEngine'
when you build in Scratchbox (Fremantle), then the problem is that your Qt dev files are out of date, to fix it, log into scratchbox and do:
> apt-get update
> fakeroot apt-get upgrade
After this you will need to re-install some of the Qt stuff
> apt-get install libqt4-gui
I had to delete my build directory (the one MOZ_OBJDIR points to in your mozconfig) afterwards too.
This post is also available in Belorussian, translated by Patricia Clausnitzer, posted on Fatcow.