Firefox sidebar auto-focus master password field and search field

#299 and its duplicates ask for auto focusing either on the master password entry field when the window opens or the search field when the vault opens. This was prevented by a [bug in Firefox](https://bugzilla.mozilla.org/show_bug.cgi?id=1324255). This Firefox bug has been fixed and released back in Firefox 60 in May 2018.

Currently in Firefox 64 I see
* When logged in Control-Shift-U opens the popup search and auto focuses the search text box
* When logged in Alt-Shift-U opens the sidebar but does not focus the search text box
* When logged out Control-Shift-U opens the popup master password prompt and auto focuses the text box
* When logged out Alt-Shift-U opens the sidebar but does not focus the master password text box

* Is it expected that the auto focus behavior does not work in the sidebar?
* If so, is it possible to do a similar autofocus step in the sidebar for folks that use that UI instead of the button popup UI?

Reproduce the logged-in and logged-out Firefox sidebar flows described in the issue, using Alt-Shift-U and checking whether the search or master-password field receives focus. Compare them with the corresponding Control-Shift-U popup flows; done means the sidebar behavior is clarified and, if supported, matches the requested autofocus behavior.

Written by the indexing model from the issue text.