Stop supporting following the system preference, but keep macOS users
able to switch to just text controls (accessibility.tabfocus=1) in the
settings.
Change the meaning of the "Use the tab key to move focus between form
controls and links" checkbox in the Firefox settings, which was
introduced in bug 1628476 to override the system setting.
The intention, I think was that this checkbox being off resulted in
"follow the system" behavior, but that didn't quite happen due to a bug
in the preferences code (this[1] won't unset the pref, because of
this[2], which means we'll just return 0).
This patch changes it so that the checkbox instead always ignores the
system setting. There will no longer be a Firefox setting (neither in
the UI nor on about:config) that means "follow system setting".
This allows us to somewhat simplify the approach compared to the
previous patch in D196110, and keep the accessibility.tabfocus working
as the source of truth without a migration.
In the future, we can think of migrating accessibility.tabfocus to a
boolean pref, which would allow us to do the cleanups to the preferences
code that D196110 did.
[1]: https://searchfox.org/mozilla-central/rev/f1532761de0b60337e42c6c3f525288a523dabef/browser/components/preferences/main.js#2252
[2]: https://searchfox.org/mozilla-central/rev/f1532761de0b60337e42c6c3f525288a523dabef/toolkit/content/preferencesBindings.js#450,483
Differential Revision: https://phabricator.services.mozilla.com/D208602
5f05578314
