By sub-menu, do you mean, for example, when you hover History, the History menu should drop down?

And you are looking for a userChrome.css hack?

I haven't looked at the code, but sometimes when you click a menu, a new attribute or class like "open" is added to the tag and that invokes a set of style rules to show the menu. If you can duplicate those rules to a :hover rule, then you may be able to emulate the effect of click with just a hover. But again, that's just a thought.

I don't think that will be possible. That pop-up menu is using Shadow DOM and that needs to be initialized via JavaScript and the pop-up menu also needs to be opened via JavaScript.

Thanks jscher2000 , yes that's right , I would like when I hover each one of top menu bar ( file edit history bookmarks ect..) , their content should drop down , that's exactly to emulate the click. I honestly don't know how to approach the matter. I've spent hours googling with no results. You posted once , something like :

1. toolbar-menubar, #menubar-items, #main-menubar ..... , I thought I could somehow do something with it , but , absolutely , no results. I am sure , it can be done , but so far , I failed.

look4

Thanks cor-el , I got it look4