Test it in Firefox-developer-edition Using just this code:

#tabbrowser-arrowscrollbox[orient="horizontal"] {

    /* Horizontal tab size */        
    &>.tabbrowser-tab[fadein]:not([style^="max-width"]),
    &>tab-group:not([collapsed]) .tabbrowser-tab[fadein]:not([style^="max-width"]) {
        max-width: 200px !important;
    }
}

For some reason, after some steps, the tab with ‘sound-muted-blocked’ becomes larger when moving other tab, I made a video showing the problem, hope someone can share a better code or a solution, the problem was in the previous developer version but with an update the problem didn’t appear anymore, I guess the developers remove some change but with the new developer-version v.142.b2 those changes were added again.

Video showing the problem: https://imgur.com/a/jeR2zN7 don’t pay attention when the YouTube page is loading, my pc is a potato.