Bug 1980856 removed both wantdropmarker support and the rule that gave
.toolbarbutton-menu-dropmarker its image, once Firefox's last consumer
stopped using it, so the New Item, Add Attachment, and New Note buttons
lost their chevrons, along with the ones in advanced search, the feed
settings dialog, and Scaffold.
Draw it with a background, since a <dropmarker> doesn't render
list-style-image, and use chevron-6 to match the quick search dropmarker
and the menu marker on our own buttons. The rule also moves out of the
toolbarbutton block, since a plain <button> can host one too.
Bug 2008041 made disabled, checked, hidden, collapsed, and selected
boolean attributes, so their value is empty and [disabled="true"] no
longer matches. Nothing sets any of them to "false", so matching on
presence alone is equivalent.
- Matching substrings from tab's title are bolded.
- Drag-and-drop functionality to reorder tabs.
- ArrowUp/ArrowDown navigate the tab's titles. Tab/Shift-Tab move
across both titles and close buttons. Enter from filter field focuses
the first tab's title. Home/PageUp focuses the filter field. End/PageDown
focuses the last tab's title. ArrowUp/ArrowDown from the filter focus
the last/first tab's title respectively. ArrowUp from the first tab or
ArrowDown from the last tab focus the filter.
