Enable loading of styles from disk - https://winaero.com/blog/enable-loading-userchrome-css-usercontent-css-firefox/ (Set at "about:config" page the option toolkit.legacyUserProfileCustomizations.stylesheets to True)
create or find userChrome.css - https://www.userchrome.org/how-create-userchrome-css.html or https://www.howtogeek.com/334716/how-to-customize-firefoxs-user-interface-with-userchrome.css/
1) Open "about:support" in Firefox
2) Click on "Profile Folder" button.
3) Steps inside folder
4) create "chrome" folder
put content of https://github.com/MrOtherGuy/firefox-csshacks/blob/master/chrome/multi-row_tabs.css to userChrome.css
In version FF 76.0.1, something was broken in again, download of latest multi-row_tabs.css helps to make work again.
create or find userChrome.css - https://www.userchrome.org/how-create-userchrome-css.html or https://www.howtogeek.com/334716/how-to-customize-firefoxs-user-interface-with-userchrome.css/
1) Open "about:support" in Firefox
2) Click on "Profile Folder" button.
3) Steps inside folder
4) create "chrome" folder
put content of https://github.com/MrOtherGuy/firefox-csshacks/blob/master/chrome/multi-row_tabs.css to userChrome.css
In version FF 76.0.1, something was broken in again, download of latest multi-row_tabs.css helps to make work again.
in FF 81.0 something changed again and required to reload latest multi-row_tabs.css from github again
