This is a Linux specific issue similar to JBIDE-1096.
Steps to reproduce:

1) activate the Javadoc view
2) close all editors (the editors that use XULRunner are important - VPE, ATF...)
3) restart Eclipse
4) try to open the VPE editor

You will get the following exception :

org.jboss.tools.vpe.xulrunner.XulRunnerException: nsIWebBrowser is not available
at org.jboss.tools.vpe.xulrunner.browser.XulRunnerBrowser.<init>(XulRunnerBrowser.java:91)
at org.jboss.tools.vpe.xulrunner.editor.XulRunnerEditor.<init>(XulRunnerEditor.java:109)
at org.jboss.tools.vpe.editor.mozilla.MozillaEditor$4.<init>(MozillaEditor.java:214)
at org.jboss.tools.vpe.editor.mozilla.MozillaEditor.createPartControl(MozillaEditor.java:214)
at org.jboss.tools.vpe.editor.VpeEditorPart.createPartControl(VpeEditorPart.java:518)
at org.jboss.tools.jst.jsp.jspeditor.JSPMultiPageEditorPart.addPage(JSPMultiPageEditorPart.java:82)
at org.jboss.tools.jst.jsp.jspeditor.JSPMultiPageEditor.createPagesForWindows(JSPMultiPageEditor.java:399)
at org.jboss.tools.jst.jsp.jspeditor.JSPMultiPageEditor.createPages(JSPMultiPageEditor.java:375)
at org.jboss.tools.jst.jsp.jspeditor.JSPMultiPageEditorPart.createPartControl(JSPMultiPageEditorPart.java:124)
at org.eclipse.ui.internal.EditorReference.createPartHelper(EditorReference.java:661)
at org.eclipse.ui.internal.EditorReference.createPart(EditorReference.java:426)
at org.eclipse.ui.internal.WorkbenchPartReference.getPart(WorkbenchPartReference.java:592)
at org.eclipse.ui.internal.PartPane.setVisible(PartPane.java:299)
...

The problem is that Eclipse restores editors/views before executing early startup code.
We can't do anything within Eclipse to solve this issue except setting the org.eclipse.swt.browser.XULRunnerPath VM argument or advising users to deactivate the Javadoc view.
I think that the best solution would be that the SWT team implements ATF's xulrunner extension point.