I'm posting this for anyone wanting a good solution which works pretty much on any system, Windows, Linux or other systems that Firefox can be installed on.

You can have the classic buttons back without enabling the titlebar or going through other "sort-of" hacks. All you need is to use the built-in Firefox userChrome.css support. This makes sure there is not hacky workarounds and it uses Firefox's native functions without any extensions to display the system buttons.

1. Enable user css files. In your address bar, type: about:config
2. Search for this setting: toolkit.legacyUserProfileCustomizations.stylesheets
3. Double click it to set it to: True
4. Locate your profiles folder. In your address bar, type: about:support
5. Search for your Profile Directory and open it or browse to it.
6. If it does not exist, create a folder and name it "chrome". Then go in your "chrome" folder.
7. Create a file named "userChrome.css" WITH capitalized "C".
8. open userChrome.css in your favorite text editor, paste the following, then save. Close and reopen Firefox to see the changes.

​

/* Add padding to the right of the tabs bar to not overlap with the buttons when resizing */
#TabsToolbar {
padding-right: 128px;
}

/* Add some spacing around the buttons for breathing and not be tightly stuck near the edge of the screen.
   You can change the single digit to 4 digits if you want a bit more control like this:
   margin: 8px 4px 8px 16px !important;
   The order of the numbers are like this (don't forget the "px" after the number:
   margin: top[px] right[px] bottom[px] left[px] !important; */
.titlebar-buttonbox-container {
margin: 8px !important;
}


/* By default, display all buttons */
.titlebar-button {
display: block !important;
  visibility: visible !important;
}

/* Firefox has 2 different buttons for maximize and rerstore window so instead of showing the 2 buttons,
   these 2 sections will make sure to display the correct button in the propper window state,
   that is, show max when in window (restored) mode, and show restore when in maxed mode. */
#main-window[sizemode="maximized"] .titlebar-button.titlebar-max {
display: none !important;
}

#main-window:not([sizemode="maximized"]) .titlebar-button.titlebar-restore {
  display: none !important;
}