Firefox Minimum Tab Width
7. February 2021
You do not have to use userChrome.css to set min-width of Firefox Browser Tabs. There is a setting for that in about:config.
Go to about:config in the browser location bar and search for setting browser.tabs.tabMinWidth in the search field on top. When you found it, click on the little pencil on the right to edit. If you change the number it takes effect immediatly, you do not have to restart firefox. I use this in combination with the quantum nox patch for multiple tab rows.