If the menu bar is hidden then press the F10 key or hold down the Alt key, that should make the menu bar appear.

• Go to "View > Toolbars" and check-mark "Menu Bar" with a click if you want to make that permanent

Note that this will make the (orange) Firefox button disappear.
You only see the Firefox button if the menu bar is hidden.

I would like the menu to be hidden until I bring it up myself by pressing alt. But in the previous versions, the menu bar remains present until you press alt again to make it disappear. In this version the menu bar disappears as soon as I click in the search bar.

That is unfortunately a bug (Bug 541844).
If the menu bar is hidden and you unhide that toolbar then you can only click menu items and not other buttons or toolbars that you have moved to the menu bar.

bug541844(fix with autohide menubar): https://addons.mozilla.org/firefox/addon/bug541844fix-with-autohide-/