We were using the same ID on two elements, which kind of messed up things everywhere
our code reasonably expected only one element of the kind to exist. We just use a
class name now.
This also cleans up #advancedPanelErrorTryAgain which worked around this issue
by using a different ID.
Differential Revision: https://phabricator.services.mozilla.com/D30298
--HG--
rename : browser/base/content/test/general/browser_bug435325.js => browser/base/content/test/about/browser_bug435325.js
extra : moz-landing-system : lando
3736102bef