https://www.howtogeek.com/333110/how-to-customize-firefox-quantum-and-remove-the-white-space-around-the-title-bar/

More to come or can search github : Here's a more comprehensive project which affects not only tabs but brings back the old "grid-style" options menu. https://github.com/axydavid/FirefoxUI/blob/master/README.md

It isn't completely possible to use the Australis design with Firefox 57, but depending on what you want, you might be able to customize the UI to look more like the old one: This project: https://github.com/wilfredwee/photon-australis lets you have the old tab bar appearance back in Firefox 57. You can remove the space around the address bar by right-clicking on the spaces, and choosing "remove from toolbar". Note the Black Theme is just a Example and can be found in Customize.

To submit suggestions for new or changed features, may I suggest: Feedback: https://qsurvey.mozilla.com/s3/FirefoxInput/

Please let us know if this solved your issue or if need further assistance.

I asked you to solve the problem by standard means, and you offer me "fly to space"

You can consider themes to adjust the toolbar area colors. There also are other adjustments available through Customize.

Customize Firefox controls, buttons and toolbars

If you want to make more significant changes to the appearance of the toolbar area, most likely you will need to apply custom style rules. Extensions are restricted in what they can do.

The following forum is all about how to achieve the appearance you want: https://www.reddit.com/r/FirefoxCSS/

Hi. It has been over 2 weeks and my problem is still not solved. Tell me, will changed the design of the browser, or will it remain a faceless design is very much reminiscent of "Microsoft Explorer"??? Maybe there is a Theme that allows you to return "Old sexy Design"? I find it difficult to use the browser. I'm close to uninstalling Mozilla from my computer. Yes, I am an esthete

Hi DadaMiki, a change to the overall theme of the browser isn't something that will be decided in two weeks or two months.

DadaMiki said

Maybe there is a Theme that allows you to return "Old sexy Design"?

How literally do you mean that? Do you want your tab bar to look exactly as it did in Firefox 29-56 ("Australis" design) with the typical colors of Windows 7? Are there any other elements you feel need to have the "Australis" appearance?

In order to curve the edges of the tabs, you will need to install some custom style rules (in other words, regular add-ons can't do it right now). It sounds as though the examples given earlier were not exactly what you're looking for but perhaps someone can put together something better for you and for other users with similar tastes.