Someone created an add-on for that, named Classic Theme Restorer. You may find the initial appearance to be a bit Windows-centric, but because it has dozens of options, you probably can make it work the way you want.

https://addons.mozilla.org/firefox/addon/classicthemerestorer/

To access CTR's Options dialog, you can use the Add-ons page. Either:

• Command+Shift+a
• "3-bar" menu button (or Tools menu) > Add-ons

In the left column, click Extensions. Then, on the right side, find Classic Theme Restorer and click the Options button. Many changes take effect immediately so you can experiment.

But don't its like the old version.

It might not look exactly like the old version. Are there particular parts of the interface that you still need to change?

Yes. Like as this video.. Watch please.. Old version.. Its more beautiful!! :)

https://www.youtube.com/watch?v=zC8M9l3BDjE

Could you be more specific about what is missing/different that you need to change? Someone who is more familiar with Mac then could assist.

Also, you should remove the "This solved my problem" from my first reply, otherwise, other volunteers are likely to ignore this question: https://support.mozilla.org/questions/1061382#answer-726828

I get it friend. I have a Mac. I'm checking here and the layout will never again as before because the browsers come with OSX layout structure, is more minimalist. I'm just saying I would love to be like before. But I'll install the add-on you suggested. Tks!!