Compact tabs in Firefox

1. Type about:config in the address bar and press Enter.
   A warning page may appear. Click Accept the Risk and Continue to go to the about:config page.
2. Find the browser.compactmode.show preference.
3. Switch the toggle to true and then close the tab.
4. Click the menu button to open the menu panel.
5. Click More tools.
6. Choose Customize toolbar….
7. At the bottom of the panel, click Density.
8. Choose Compact (not supported) from the menu options.

   
   

9. Click Done. 

From: https://support.mozilla.org/en-US/kb/compact-mode-workaround-firefox