Bug 1538366 - don't show accounts button in popups and ensure it gets an arrow in the overflow panel, r=MattN

Differential Revision: https://phabricator.services.mozilla.com/D24575

--HG--
extra : moz-landing-system : lando

browser/base/content/browser.xul

@@ -1087,7 +1087,7 @@
                        tooltiptext="&libraryButton.tooltip;"
                        label="&places.library.title;"/>
 
-        <toolbarbutton id="fxa-toolbar-menu-button" class="toolbarbutton-1 badged-button"
+        <toolbarbutton id="fxa-toolbar-menu-button" class="toolbarbutton-1 badged-button chromeclass-toolbar-additional subviewbutton-nav"
                        onmousedown="gSync.toggleAccountPanel('PanelUI-fxa', event)"
                        onkeypress="gSync.toggleAccountPanel('PanelUI-fxa', event)"
                        consumeanchor="fxa-toolbar-menu-button"