Correction: it seems it wasn't a theme add-on that made the corners of the tabs rounded, but FF itself. That makes it even worse! Plus: many of the other add-ons didn't work anymore either in FF Quantum. And I'll be honest: THE reason why I keep using Firefox is the add-ons. So, I went back to FF 56. For me, all those problems must be solved, guys, before I'm going to upgrade again. And if they are not solved, I actually do not see a reason why I shouldn't switch to Chrome, eventually.

See:

• https://github.com/wilfredwee/photon-australis

Thanks, cor-el. When the problems with the add-ons are solved I might just use your solution and upgrade to Quantum again. But I wonder whether those problems will be solved. And Mozilla folks: my main comment remains: get a better stylist. Yours really suck(s).

Going back : unfortunately 56.0.2 is no longer safe to use for every day use as has security issues, but 52.5.0 ESR is! It will continue to get security updates until May 2018, and you can download and install it from this page: https://www.mozilla.org/en-US/firefox/organizations/all/ Note : Legacy Extensions will be deleted or removed in any version update after May 2018.

Fyi : https://www.howtogeek.com/333230/why-firefox-had-to-kill-your-favorite-extension/ How about looking at these 2 URL's first : https://www.howtogeek.com/333110/how-to-customize-firefox-quantum-and-remove-the-white-space-around-the-title-bar/ and The name is not descriptive enough : https://www.howtogeek.com/332449/whats-new-in-firefox-quantum/

More information will come out slowly. New extensions are being made and some older ones being updated that are tracked here : http://bit.ly/2j0Anlj

Thanks for the info, Pkshadow. I'll look into into it one of these days.

UPDATE

I waited until all but one of the functional add-ons had become compatible with Quantum and then upgraded to v. 59. Fortunately, my favorite theme add-on kept working, too. That was not the case in v. 58.

And a very big improvement in Quantum for me is this. The Dutch news website nu.nl would refuse to release its videos to FF, even with the Adblock disabled and all FF settings at default. That problem is now solved and I'm assuming it is due to Quantum.

I still think it is a pity that the curved tabs have disappeared, because the Photon Australis option does a rather poor job: no border line and a white background of the active tab. I would strongly suggest to simply give the FF customers a choice: straight or curved tabs (or more options). Should be a piece of cake for the makers and will definitely result in more happy campers.

HI unfortunately your suggestion is like making 2 versions. So a Standard or a Automatic. That is the reason car companies went broke and most just make automatics now. It is not practical for a non-profit company to do so. Besides, with over 235 million ways to make changes to Firefox with usserChrome it defeats the purpose. Need to learn to write code or find better code. USSER CHROME URL'S

• https://www.howtogeek.com/334716/how-to-customize-firefoxs-user-interface-with-userchrome.css/
• http://techdows.com/2017/09/classic-theme-restorer-userchrome-css-modify-firefox-57-photon-ui.html
• https://github.com/Aris-t2/CustomCSSforFx/issues/1
• https://www.accessfirefox.org/Theme-Font-Size-Changer.php
• https://www.userchrome.org/
• https://www.accessfirefox.org/Firefox_Accessibility_Themes.php
• https://www.userchrome.org/what-is-userchrome-css.html
• http://kb.mozillazine.org/UserChrome.css
• https://github.com/axydavid/FirefoxUI/blob/master/README.md
• https://github.com/wilfredwee/photon-australis
• https://www.reddit.com/r/FirefoxCSS/

Note not all code in one place or is it the total of sites, google it, learn it.

To submit suggestions for new or changed features, may I suggest: Feedback: https://qsurvey.mozilla.com/s3/FirefoxInput/ or https://discourse.mozilla.org/c/add-ons

Please Mark this as Solved with what ever answer you want to as this is not a Firefox problem or issue. This is a matter of preferences only.

FrankCo said

I still think it is a pity that the curved tabs have disappeared, because the Photon Australis option does a rather poor job: no border line and a white background of the active tab.

If I recall correctly, the author of Photon Australis posted 3 variations to match the 3 built-in themes (default, light, dark), so you could try each and see whether one of them suits you.

One of the reasons it is difficult to reproduce the old design is the inefficiency of using overlaid bitmap images for the little curvy bits. The Photon Australis style rules use SVG, a drawing language which may be faster but its hard to edit, at least for me.

If you can live without the flared lower part of the active tab, so just the "shoulders" of the tabs are curved, I posted an example style recipe last night in another thread:

https://support.mozilla.org/questions/1210148#answer-1091800

I would strongly suggest to simply give the FF customers a choice: straight or curved tabs (or more options). Should be a piece of cake for the makers and will definitely result in more happy campers.

I don't think it would be a piece of cake. But a volunteer could develop and submit it for consideration.

I wanted to post this message earlier, but was too busy. Improving on instructions by others and helped by the knowledge of web CSS that I already had, I managed to create a much improved interface. See the below images. The 'How To' is posted on https://github.com/FrankConijn/Firefox-Quantum-Rounded-Tabs. It also has a nice hoover effect.

@jscher2000: Thanks for the help, but I like my tabs a bit better.