All new Mozilla Firefox versions come with tabs in titlebar to save space but there used to be an option "Tabs on Top" in toolbar context menu which allowed you to toggle between tabs on top and tabs on bottom. So you could have the tabs below the toolbar using that option.
If you have upgraded to the new version of Firefox but want to have the tabs on bottom, following tutorial will help you.
You can get the tabs back on bottom with the help of a secret preference present in about:config page:
1. Open Firefox and type about:config in the addressbar and press Enter. It'll confirm, click on I'll be careful, I promise! button.
2. Now type ontop in the Filter box and look for following boolean key:
3. The entry "browser.tabs.onTop" would be set to true. Double-click on it or right-click on it and select "Toggle". It'll set its value to false.
4. That's it. It'll immediately put tabs on bottom. Enjoy.