Enter about:config in the URL bar and set browser.tabs.tabmanager.enabled = false, then restart the browser.

Setting the browser.tabs.tabmanager.enabled value to false no longer works. It worked a few days ago, but now the List All Tabs button is back and no longer responds to that configuration setting.

I have also tried the profile/chrome/userChrome.css fix but that doesn't work either. Instead of having users jump through hoops to force a UI element to be optional like everything else on the UI, how about you just make it optional like the rest? Why does List All Tabs need to be the only Toolbar Customization element that is greyed out so users can't alter it outside hacking some configs manually?

The "List all tabs" (Tab Manager) button will still appear automatically with the pref set to false if you open a lot of tabs and the tab scroll buttons appear.