Sorry, but the Quantum Firefox 57 version killed a lot of older, legacy extensions and themes, as far as what they are allowed to change in the user interface. Some features have been replaced with the new WebExtensions format, but most just aren't possible to replicate in WE format and are gone forever. That said, it has been so long and 20+ new versions have been released, along with my never having used TabMix Plus, that I can't help you with particulars.

Many features that were in TabMix Plus can be added to Quantum Firefox via the use of an optional user created userChome.css file. See this as it relates.
https://github.com/aris-t2/customcssforfx

https://www.userchrome.org/
https://www.userchrome.org/how-create-userchrome-css.html
https://www.userchrome.org/help-with-userchrome-css.html

Or switch to a different web browser that has the features that you want.

Or switch to a different web browser that has the features that you want. </blockquote>

OK thanks i think i got the picture. Problem is none has the features i want, so i guess i will stay forever in Firefox v56.0.2 that was perfect.

Some(a lot recently) websites have already started to not functioning correctly on that, so in the future it will be even worse but that will be a problem of the future for me.

I find it idiotic from Mozilla to migrate to something that abandons and does not support so many great features. Making steps back in certain areas(no doubt it has made steps forward to others) is a thing i will never understand.

I have some questions and comments:

a) When i hover with the mouse(and NOT click into it) to a tab for a specific amount of time I can set, then tab is being selected. That can be done with the powerful Tab Mix Plus addon in old Firefox.

Is the purpose of hovering instead of clicking to avoid having to find the previous tab and make it active again?

Generally speaking, a WebExtension can't take an action merely because a user hovered over a built-in control. However, it could provide a separate list of tabs to replicate this "peeking" (temporary activation) function.

b) When i right click in the page i had the option to save all images of this tab/of all tabs in the left or right and saving them in ANY FOLDER of my preference. I can do that with the Save Images 1.3.0 addon in old Firefox.

The WebExtensions APIs limit disk access so extensions can only save automatically in your downloads folder or subfolders of that folder.

(Note that a WebExtension can coordinate with a separately installed companion program as a workaround to this limitation.)

c) There was a display closed tabs list button that if you click it ONCE it makes the most recently closed tab to load and when you click the tiny ↓near it, you had a list of recently closed tabs you could choose anyone to reload.

So you're saying that the built-in menu paths are too long? For example:

• (menu bar) History > Recently closed tabs > {list}
• optional History toolbar button > Recently closed tabs > {list}

There might be an extension which gives you a more direct route, but I've never gone looking for one.

d) How to make buttons more customizable(bigger) and with colors and themes? That minimalist design sucks for my taste and i want it to be fully configurable. I am using the beaufitul Noia theme for buttons etc in my old Firefox.

A userchrome.css file.

Firefox 56.0.2 is not secure and should no longer be used on the internet. You may want to look at Waterfox for a more updated descendant of Firefox 56 if you are unable to live with later changes.

Is the purpose of hovering instead of clicking to avoid having to find the previous tab and make it active again?

To avoid clicking. :) It's MUCH faster this way to guide through tabs. Trust me i use it for ages so it's a lot more convenient.

The WebExtensions APIs limit disk access so extensions can only save automatically in your downloads folder or subfolders of that folder.

Bummer. From all i hear this WebExtensions is all about limitations. I do wonder why they migrated to it.

So you're saying that the built-in menu paths are too long? For example:

• (menu bar) History > Recently closed tabs > {list}
• optional History toolbar button > Recently closed tabs > {list}

That is 3 clicks each one. What i say was 1 click for the latest tab and 2 cliks for an older one. Again faster. And since it's a thing one does countless time it saves a lot of time.

A userchrome.css file.

If only i knew how to manupulate colors buttons etc with this. I mean i do not want to learn a new programming language to have a nice beautiful theme on the buttons etc. In old Firefox there were addons/themes for this. You added them and that was it! In new Firefox it seems the user has to use workarounds and learn programming language stuff. Well no, this logic is beyond me so will not try to use it....

Firefox 56.0.2 is not secure and should no longer be used on the internet. You may want to look at Waterfox for a more updated descendant of Firefox 56 if you are unable to live with later changes.

Is Waterfox safe? Does it support old addons?

Thanks anyway, for the reply. :)