What do you mean by "the header"? Are you referring to something on a website or part of the browser?

Hi there, I was in the firefox web browser section looking for answers about how to increase the font size of the bookmarks bar. I have already increased the font size of the webpages but the bookmarks are tiny. Is there a way to make them larger? Cheers

You have to create a file called userChrome.css. This code may do what you want: https://support.mozilla.org/en-US/questions/1348994

You can set layout.css.devPixelsPerPx to 1.0 (default is -1) on the about:config page. Adjust its value in 0.1 or 0.05 steps (1.1 to enlarge or 0.9 to reduce) until icons or text looks right. Make sure not to use values too large or too small.

• modifying layout.css.devPixelsPerPx affects user interface and webpages (global zoom)

Firefox has a Zoom section in Settings to set the default zoom level for webpages.

• Settings -> General -> Language and Appearance -> Zoom

You can open the about:config page via the location/address bar. You can click the button to "Accept the Risk and Continue".

• https://support.mozilla.org/en-US/kb/about-config-editor-firefox