Why use userChrome.css code to hide the Menubar?

Having the Menubar be hidden is a default feature in Firefox for quite awhile now, and the "switch" is available in multiple places in the UI, along with the Alt & F10 keys being able to display the Menubar at will. It shouldn't cause the Titlebar buttons to quit working whether the Menu Bar is showing or not showing.

As far as the Windows Title Bar buttons - Minimize, Maximize, and Close buttons - not working, look to your customizations in Firefox for causing that to occur. Could be a slightly broken extension or some "user code". Going back to Firefox 40-something I had an issue with F11 Fullscreen not working, and the cause was a UserSyle (for Stylish) that needed to be tweaked a bit to get rid of that fault. Unfortunately I don't recall the line of code that caused that fault, as it was occurring in one of my lesser used Profiles.

Thanks for your reply

If I use the right-click menu to hide the menu the buttons are working. But if I restart the browser the menu bar appearce again, I want to disable it by default. It seems to be a problem with the firefox version. I'm using the portable Version from portable apps, if I use a normal firefox version the menu bar stays disabled.

Is there another way to hide the menu bar?

View > Toolbars -> Menu Bar.

Thanks but same behaviour like the right click menu, if I disable the menu bar it apepars again after a restart of the browser.

How about when launching Firefox in the Firefox SafeMode? What happens then?

Help > Troubleshooting Information -> Restart with Add-ons disabled ... - which does a lot more than disable add-ons

Or - with Firefox not running, hold the Shift when you double-click the Firefox desktop shortcut.

Hey, sorry for the delayed answer. If i launch Firefox in the SafeMode the menu bar is hidden, it doesn't matter if I active the menu bar befor restarting, the menu bar is alsways hidden. But Safe-Mode is not an Option for the normal use. I've no custom plugins/add-ons installed.