To see whether an extension, theme, or custom setting could be a factor, could you try a new Firefox profile. In addition to bypassing current add-ons and custom settings, it should have completely fresh settings databases and a fresh cache folder. You can use the profile manager to create profiles, and switch between them for testing or other purposes. This article describes how to start Firefox in the Profile Manager (make sure to completely exit Firefox first): [[Use the Profile Manager to create and remove Firefox profiles]] To avoid data loss, I recommend accepting the default location for the new profile. Although it is easy to delete entire profiles, I suggest a backup before doing that. After you create and start up in the new profile, is there any difference in how the site works?