Go to about:support in FF, then by the ‘Profile Folder’ setting (about 8th from the top) click the ‘Open Folder’ button. Create the ‘chrome’ subfolder, if you don’t already have one. Inside that folder create a file called ‘userChrome.css’, inside that file paste the text from https://github.com/andreicristianpetcu/UserChrome-Tweaks/blob/09fa38a304af88b685f4086bc8ea9997dd7db0fd/tabs/multi_row_tabs_firefox_v57.css
Update that text to replace ‘.tabbrowser-tabs’ with ‘#tabbrowser-tabs’. I didn’t have to worry about the ‘.tabbrowser-tab’ entries, it still seemed to work.
Restart FF and you should have multi row tabs