This change is made for the following reasons: - Similarity between new "Extensions" menu item, which shows up instead of "Extensions and Themes" when the Extensions Button is hidden. - Consistency with about:preferences, which shows "Extensions & Themes" (this was introduced in bug 1483335 and was not touched when the menu labels were renamed in bug 1698931). - Consistency with mobile (bug 1885561) This effectively reverts the rename of bug 1698931. Differential Revision: https://phabricator.services.mozilla.com/D248162