That option has been gone from Firefox since Firefox 29 came out, although there was an extension available until Firefox 57 was released which added that feature back into Firefox. Now that can be done via userChrome.css codes.

Brief explanation about userChrome.css and how to use it. https://www.userchrome.org/ Look for the needed code here: https://www.reddit.com/r/FirefoxCSS/

Why is there no option to move tabs below address bar?

Because some developers decided, that they don't want to test nor maintain it every day anymore. They'd rather be spending that time working on things that are more broadly-relevant and have greater impact (snappier UI, new features, bug fixes, etc).

• It is undeniable that additional lines of code requires maintenance over time. It costs time and money.
• Extra code also makes it harder to learn the project, and requires extra understanding and navigating.
• It is harder to refactor, simplify, or optimise a program when it is bogged down by spaghetti code.

is there code to move the tabs below the window and just above the task bar?