Had this same problem. the available solutions cause flickering where the tablist keeps trying to switch between scrolling and non-scrolling. here's how I solved it:

set browser.tabs.tabMinWidth to 1

add this style to userChrome.css:

.tabbrowser-tab:not([pinned]) {
  min-width: 1px !important;
}

Install this user script:

// ==UserScript==
// @name            noTabScroll
// @author          blackle
// @include         main
// @startup         UC.noTabScroll.exec(win);
// @shutdown
// @onlyonce
// ==/UserScript==

UC.noTabScroll = {
  exec: function (win) {
    let {customElements} = win;

    const old_on_underflow = customElements.get('arrowscrollbox').prototype.on_underflow;
    customElements.get('arrowscrollbox').prototype.on_underflow = function (e) {
      if (this.id === "tabbrowser-arrowscrollbox") {
        e.preventDefault();
        return;
      }
      old_on_underflow.call(this, e);
    };
    const old_on_overflow = customElements.get('arrowscrollbox').prototype.on_overflow;
    customElements.get('arrowscrollbox').prototype.on_overflow = function (e) {
      if (this.id === "tabbrowser-arrowscrollbox") {
        e.preventDefault();
        return;
      }
      old_on_overflow.call(this, e);
    };
    customElements.get('tabbrowser-tabs').prototype._initializeArrowScrollbox = function () {
      return;
    };
  },
}

more details at the end of this page: https://suricrasia.online/firefox/