Attached is the console output. When trying to embed a webkit browser in my gtk+ application I get the same errors.
** (epiphany-browser:12280): DEBUG: NP_Initialize
** (epiphany-browser:12280): DEBUG: NP_Initialize succeeded
** (epiphany-browser:12280): DEBUG: NP_Initialize
** (epiphany-browser:12280): DEBUG: NP_Initialize succeeded
** (epiphany-browser:12280): DEBUG: NP_Initialize
** (epiphany-browser:12280): DEBUG: NP_Initialize succeeded
** (epiphany-browser:12280): DEBUG: NP_Initialize
** (epiphany-browser:12280): DEBUG: NP_Initialize succeeded
** (epiphany-browser:12280): DEBUG: NP_Initialize
** (epiphany-browser:12280): DEBUG: NP_Initialize succeeded
** (epiphany-browser:12280): DEBUG: NP_Initialize
** (epiphany-browser:12280): DEBUG: NP_Initialize succeeded
** (epiphany-browser:12280): DEBUG: NP_Initialize
** (epiphany-browser:12280): DEBUG: NP_Initialize succeeded
** (epiphany-browser:12280): DEBUG: NP_Initialize
** (epiphany-browser:12280): DEBUG: NP_Initialize succeeded
(epiphany-browser:12280): GLib-CRITICAL **: g_hash_table_foreach: assertion `hash_table != NULL' failed
(epiphany-browser:12280): GLib-CRITICAL **: g_hash_table_insert_internal: assertion `hash_table != NULL' failed
(epiphany-browser:12280): GLib-CRITICAL **: g_hash_table_lookup: assertion `hash_table != NULL' failed
*** NSPlugin Wrapper *** ERROR: no valid NPP -> PluginInstance mapping found
(epiphany-browser:12280): GLib-CRITICAL **: g_hash_table_lookup: assertion `hash_table != NULL' failed
*** NSPlugin Wrapper *** ERROR: no valid NPP -> PluginInstance mapping found
(epiphany-browser:12280): GLib-CRITICAL **: g_hash_table_lookup: assertion `hash_table != NULL' failed
(epiphany-browser:12280): GLib-CRITICAL **: g_hash_table_insert_internal: assertion `hash_table != NULL' failed
(epiphany-browser:12280): GLib-CRITICAL **: g_hash_table_insert_internal: assertion `hash_table != NULL' failed
(epiphany-browser:12280): GLib-CRITICAL **: g_hash_table_find: assertion `hash_table != NULL' failed
(epiphany-browser:12280): GLib-CRITICAL **: g_hash_table_insert_internal: assertion `hash_table != NULL' failed
(epiphany-browser:12280): GLib-CRITICAL **: g_hash_table_lookup: assertion `hash_table != NULL' failed
*** NSPlugin Wrapper *** ERROR: no valid NPP -> PluginInstance mapping found
(epiphany-browser:12280): GLib-CRITICAL **: g_hash_table_lookup: assertion `hash_table != NULL' failed
epiphany-browser: /build/buildd/nspluginwrapper-1.2.2/src/npw-rpc.c:1225: do_recv_NPObject: Assertion `npobj != ((void *)0)' failed.
*** NSPlugin Viewer *** ERROR: NPN_GetProperty() wait for reply: Connection reset by peer
*** NSPlugin Viewer *** WARNING:(/build/buildd/nspluginwrapper-1.2.2/src/npw-viewer.c:898):invoke_NPN_GetValue: assertion failed: (rpc_method_invoke_possible(g_rpc_connection))
Aborted
*** NSPlugin Viewer *** WARNING: unhandled variable 18 (<unknown variable>) in NPN_GetValue()
*** NSPlugin Viewer *** WARNING:(/build/buildd/nspluginwrapper-1.2.2/src/npw-viewer.c:898):invoke_NPN_GetValue: assertion failed: (rpc_method_invoke_possible(g_rpc_connection))
epiphany-browser
Sorry, the web page I was trying to visit isn't public, but it just uses Ext JS version 3. All other web pages appear to work.