I removed onerrorcommand as part of bug 255731 at Neil's suggestion (bug 255731 comment 7), but I now think that was not the right thing to do. autocomplete still tries to fire it http://bonsai.mozilla.org/cvsblame.cgi?file=mozilla/xpfe/components/autocomplete/resources/content/autocomplete.xml&rev=1.136&mark=869,970#869 and some consumers still try to set an actual handler http://lxr.mozilla.org/seamonkey/search?string=onerrorcommand
Created attachment 246870 [details] [diff] [review] patch
Comment on attachment 246870 [details] [diff] [review] patch Hewitt removed support for onerrorcommand in revision 1.67 for some reason. If you really want to restore it you would also have to undo those changes.
He switched from using fireInit, fireTextCommand, fireErrorCommand and fireTextRevert methods to just a single _fireEvent method... but I don't see anything that actually removed support.
Yes, but that uses the attribute, not the field.