Firefox: How to reduce height of bars

Solution 1:

OPTION ONE:

First, go to about:config in your URL bar.

Then, search for browser.compactmode.show and change the value to true .

Finally, go to the Hamburger-Menu > More-tools > Customize-toolbar and at the bottom of the page, click Density and select Compact.

OPTION TWO (recommended):

First, go to about:config in your URL bar.

Then, search for layout.css.devPixelsPerPx

Finally, double click on the value and change it to less than 1.0. After you set the value, press ENTER to apply the changes. For example, you can use 0.75 or if that is too big, you could try 0.5.

Solution 2:

If you are referring to the header bar in Firefox v90 then there are facilities to remove it.

• Click on the 3 lines (so called hamburger) icon to open Firefox menu.
• Click More Tools then "Customise …” menu option to open new Customise Firefox tab.
• Finally un-check the “Title Bar” check-box in the bottom-left.

reference