Firefox: hide navigation toolbars only when top menu is inactive

[infinity0]

Hiding navigation toolbars is nice most of the time, however sometimes you have several tabs open either on purpose or by accident, and want to close some of them. (e.g. #101, #105, #266)

But going back into the webapp-manager to edit the "hide navigation bar" option is inconvenient especially when you must do this every single time there are extraneous tabs.

In fact Firefox already has a nice feature that hides the menubar when it's not needed, and shows it only when it's needed - the Alt button.

This PR hooks the rest of the navigation bar onto this mechanism, so pressing Alt will now toggle the menubar+navbars together.

On my version of Firefox (latest) it's not necessary to explicitly style #identity-box, #tabbrowser-tabs since they are inside the other elements, I'm not sure of the purpose of the original code. But that's what this PR & code review is for, to discuss this. :)

[infinity0]

Hm, the behaviour for the tabs isn't the most intuitive. You can right-click on them for the usual context menu but nothing else works (dragging, closing) because Alt puts the focus on the menu bar. I could try to fix this with some javascript in userChrome.js however if you think that's a good idea.

[infinity0]

Hm, the behaviour for the tabs isn't the most intuitive. You can right-click on them for the usual context menu but nothing else works (dragging, closing) because Alt puts the focus on the menu bar. I could try to fix this with some javascript in userChrome.js however if you think that's a good idea.

The latest commit fixes these issues by only hiding the navbar when the user isn't otherwise interacting with it via :hover, :focus or :active, in addition to checking that the menu bar is inactive. Dragging and closing the tabs now works! No JS required, CSS-only solution.