- Go to about:config, enable config
toolkit.legacyUserProfileCustomizations.stylesheets
. - Go to about:support >
Profile Directory
>Open Directory
. Firefox profile directory will be openned in new window. - In your profile directory, create folder
chrome
if not exist. - Go to folder
chrome
, create fileuserChrome.css
. - Copy content from multi-row_tabs.css
into
userChrome.css
. - Restart Firefox.