Focus sidebar on opening in Firefox

[matatk]

Opening the browserAction pop-up via the shortcut key focuses the pop-up. This doesn't happen for the sidebar on Firefox. There's a Bugzilla bug on that, and one of the comments explains how it might be done on the extension's side.

(Opera doesn't support an API to open the sidebar so this isn't applicable there, because I didn't want to implement a separate command for opening the pop-up and sidebar, so there isn't a way to open the sidebar via keyboard directly.)