root cause: Eclipse 3.4.1 is not compatible with some versions of xulrunner (will be fixed in Eclipse 3.5)
If the IDE does start up and you select Help->Welcome you get an SWT error. Also some of the buttons, e.g. in the project wizard can not be selected or submitted via mouse pointer.A workaround is to disable the Welcome page display as follows:
# cd YOUR-WORKSPACE-DIR
# rm -rf .metadata
# mkdir -p .metadata/.plugins/org.eclipse.core.runtime/.settings
# echo showIntro=false > .metadata/.plugins/org.eclipse.core.runtime/.settings/org.eclipse.ui.prefs
To permanently allow you to work with the welcome and html-pages, you will need to download and install the gdk_eclipse_fix with the Package Manager.
You may still have some buttons which can be selected with the mouspointer but not submitted by pressing a mouse-key. In that case, the workaround is to press the keyboard "TAB" to navigate and "Enter/Return" to submit those buttons.