See e.g. the current thingymajig with dom.indexeddb.enabled, or bug 1120236, or bug 1121706, or ... There are a lot of cases where (hidden) prefs we have break lots of UI. Or where non-default changes to customizable UI things (like which buttons exist on startup, or go in a panel, or...) break stuff. Loop had a fight with this, some of the devedition things had a fight with this, now the search bindings are having a fight with it... It would be interesting if we had a way of automatically testing some basic functionality (startup without errors, do a web search, navigate to some of the alexa top 100 pages, fill in a login form, ...) under varying UI/pref conditions to try to root out more of these things, especially where they are severe issues. Kind of like MattN's screenshot tool, but for functional issues.