Support for tabs filling available width

[yokoffing]

Is your feature request related to a problem?

I don't like the default tab width.

The solution I'd like

Tabs to fill the available width of the tab bar, similar to how the previous Safari UI worked. 1 tab takes up 100% of the tab bar, 2 tabs take up 50% each, 3 tabs 33%, etc.

Screenshots or video

Alternatives I've considered

Similar to https://github.com/MrOtherGuy/firefox-csshacks/blob/master/chrome/tabs_fill_available_width.css, but dropping this into my userChrome doesn't work when using Firefox UI Fix.

Additional context

[black7375]

Currently applicable as follows:

.tabbrowser-tab[fadein]:not([style^="max-width"]) {
  --tab-max-width: 100vw;
}

I will put it as an official option in 7.0.

[black7375]

Try to use v7.0.0.