Firefox 131 is out, and with it arrives a change to the Tab overview menu: “a new, refreshed icon”.
There has been some outcry on social media, since the redesign came with an undesirable change for some - the button is no longer able to be hidden by default.
browser.tabs.tabmanager.enabled = false
Sorry: https://bugzilla.mozilla.org/show_bug.cgi?id=1916622