if you still use kde3, open synaptic, look for kdelibs, select it, try to uninstall it and see what kde3 applications it will attempt to pull down with it.
basically if you remove kde3, all applications that depends on kde3 will be removed, firefox is not a kde3 application and it wont be removed, virtualbox is not a kde3 application and it wont be removed. Kaffeine, k3b,kopete, ark, konqueror among others will be.