See:

• https://support.mozilla.org/en-US/kb/forum-response-tabs-below-url-bar

See also:

• https://www.userchrome.org/what-is-userchrome-css.html
• https://www.userchrome.org/how-create-userchrome-css.html

In Firefox 69 and later you need to set this pref to true on the about:config page to enable userChrome.css and userContent.css in the chrome folder.

• toolkit.legacyUserProfileCustomizations.stylesheets = true
• https://www.userchrome.org/firefox-changes-userchrome-css.html

Thanks, still baffles me why they WIPE this @ EVERY update! Guess I am lucky this time moz did not erase current session! Yet leave behind the transparent empty bar. Will wade threw that long thread and try to figure out which part they clobbered WITHOUT permission! about.config is still set to true, so they clobbered chrome? Thanks again!

Wow this forum is crazy. Keeps trying to sign me in to the wrong account. Thanks I replaced my userchrome file and it fixed this issue. I was reluctant since I could have sworn userchrome was used for more than moving tabs. We'll see. Hopefully I did not just clobber something else. Thanks again.

If your current code to get the tabs on bottom isn't working in Firefox 71 then you can add this rule to your userChrome.css or edit an existing rule for #TabsToolbar and add display: block !important;

 #TabsToolbar {
  display: block !important;
 }