Let's say I'm in Firefox. I grab a corner of the window and shrink the window in half. I click the green light. The window maximizes, but the menu bar disappears. If I double click the Firefox window, it minimizes to the dock.
I'm trying to lock the menu bar in place so it never disappears.
The green button by definition activates full screen mode and by definition the menu bar in full screen mode only appears when you move the cursor to the top of your screen. This is touted as a "feature" of full screen mode because it allows the app to use every available pixel on the screen for its display. By-the-way you might give full screen mode an honest try. Once you become accustomed to it, and learn to switch between full screen apps using App Switcher (programmable on your mouse) or a three finger swipe on the trackpad it is really pretty useful. I find it particularly useful on my 13" Macbook Pro when it is not connected to the 27" Thunderbolt Display.
Short of reverting to an earlier version of macOS the best (only?) alternative is the "zoom" feature suggested by DK. It is not the same as full screen and works differently in different apps but it does work reliably.
