User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_9_5) AppleWebKit/600.3.18 (KHTML, like Gecko) Version/7.1.3 Safari/537.85.12 Steps to reproduce: Tried to use these preferences in about:config: accessibility.typeaheadfind.autostart accessibility.typeaheadfind.enabletimeout accessibility.usebrailledisplay accessibility.usetexttospeech application.use_ns_plugin_finder autocomplete.grab_during_popup autocomplete.ungrab_during_mode_switch browser.chrome.toolbar_style browser.download.debug browser.EULA.version browser.popups.showPopupBlocker capability.policy.default.SOAPCall.invokeVerifySourceHeader dom.idle-observers-api.enabled dom.mozContacts.enabled dom.workers.maxPerDomain intl.fallbackCharsetList.ISO-8859-1 layout.frame_rate.precise media.audio_data.enabled middlemouse.openNewWindow network.http.bypass-cachelock-threshold prefs.converted-to-utf8 print.always_cache_old_pres print.print_extra_margin print.print_paper_size profile.confirm_automigration profile.manage_only_at_launch profile.migration_behavior profile.migration_directory profile.seconds_until_defunct security.ask_for_password security.checkloaduri security.password_lifetime security.xpconnect.plugin.unrestricted toolbar.customization.usesheet toolkit.asyncshutdown.timeout.crash ui.key.saveLink.shift viewmanager.do_doublebuffering Actual results: They have no effect. Expected results: The preferences in about:config should be updated as preferences are added and removed during evolution. For the above preferences, they actually have been removed from source code (checked the source code) in some previous versions but they are still in about:config. Even, for some of them I found some documentation explicitly stating that they removed the feature. However, they are still available in about:config. It is not possible for everyone to go and check the source code to see if there is a bug, or h/she is doing something wrong, or this is just an intentional change while about:config has not been updated.
Note that some of these (e.g. browser.chrome.toolbar_style, middlemouse.openNewWindow, ui.key.saveLink.shift, ...) are in use in comm-central. There are also some cases of code in the tree that uses string concatenation to remove a common prefix from a set of prefs, which makes those prefs a pain to search for (e.g. browser.urlbar.delay). I didn't find any examples in your list, but I didn't check them all (and, well, like I said, they're a pain to search for).
As noted, a bunch of these (also e.g. usesheet, the a11y prefs) are in use in comm-central. They could be moved there, but that would require more investigation for the individual cases you list. I can mentor this, but I don't have time to actively develop on this myself.
Status: UNCONFIRMED → NEW
Component: Untriaged → General
Ever confirmed: true
Summary: Some preferences with no effect exist in about:config → Remove or move (to suite) defunct prefs listed in our m-c prefs/all.js files (and therefore about:config)
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