Use a URL path to point to the image file as documented in the following site: <a href="https://developers.arcgis.com/web-appbuilder/api-reference/app-configuration.htm" target="_blank">https://developers.arcgis.com/web-appbuilder/api-reference/app-configuration.htm</a>- For example, change the configuration from "images<a href="file://logo.jpeg" target="_blank">\\logo.jpeg</a>" to "images/logo.jpeg".By default, Mozilla Firefox does not allow UNC paths even though UNC can be regarded as URL paths: <a href="http://kb.mozillazine.org/Links_to_local_pages_don" target="_blank">http://kb.mozillazine.org/Links_to_local_pages_don</a>'t_work#UNC_Paths. This is a security issue, and Firefox does not allow UNC for security reasons.