See also:

bug 1722896
bug 1484955
bug 1584585

and the discussion in https://phabricator.services.mozilla.com/D122335#inline-677124 and https://phabricator.services.mozilla.com/D122335#3988172

All this duplication is confusing. It would be better to move the network and cert error pages into toolkit and make them responsive enough that consumers would, by and large, not need to touch things anymore. I thought we already had this on file, but I can't find it, so in the spirit of better dupes than not having things tracked anywhere, here it is.