After hunting down similar threads on gtk+ problems, I finally got it solved (and by reading documentation on www.gtk.org,
learned a lot ;-)
I'll post the solution, hopefully it can be usefull for other newbies.
Thinking (please, correct me if I'm wrong, I merely guessing) that the apps that did not start up anymore must depend on an older version of gtk+, I wanted to reinstall gtk+-1.2 from the Redhat package CD. But for that I needed glib-1.2, which was exactly the problem why for example firebird didn't want to start. When glib-1.2 was installed (from Redhat CD) all was fine! (and as it turned out, gtk+-1.2 was already installed).
So I'm a happy fluxer again!