how can I change the bottom border of the search autocomplete popup in firefox to make it black, solid and 1-pixel width?
By default there is no bottom border. See this screenshot http://postimg.org/image/jjnf66zwv/97dbdf31/.
Moved to Support since this isn't a theme issue.
memeplex wrote: how can I change the bottom border of the search autocomplete popup in firefox to make it black, solid and 1-pixel width?
The exact CSS would depend on what other rules are applying to that element.
Note, the element structure is for Firefox 34; there's been at least one change since then but I don't believe it impacts what you're trying to do.
// This hbox displays the current chosen (default) search engine.
// This hbox and its contents are hidden if no text has been entered by the user.
// This tree displays suggested autocompletion values for the search text entered by the user.
// This tree and its contents are hidden if no text has been entered.
// This hbox displays the current text for which a one-off search would be executed.
// This description contains the icons of individual search engines to pick for a one-off (one-time) search.
// The description and its contents are hidden if all alternate engines are disabled.
// In this example, we show the DOM for two rows of icons, five engines in the first row and two in the second.
// engines are available (or if already installed).
// See also (Fx36) Bug 1106432 which changes the representation of .addengine-item to show a badge on it.