Add code to the userChrome.css file below the default @namespace line.

• http://kb.mozillazine.org/userChrome.css

The customization files userChrome.css (interface) and userContent.css (websites) are located in the chrome folder in the Firefox profile folder.

• http://kb.mozillazine.org/Editing_configuration

Adjust the font size to suit your needs (in px or pt).

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); /* only needed once */

#personal-bookmarks .bookmark-item .toolbarbutton-text {
 font-size: 9pt !important;
}

• http://www.handsonweb.com/styles/fonts2.html

Ik begrijp niet wat er bedoeld word en hoe ik dit moet uitvoeren.English ok,maar niet technical english.

You can use this button to go to the Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder)

You can create the chrome folder (lowercase) in the profile folder with the random name (xxxxxxxx.default) and place a file userChrome.css in it.
Make sure that you select "All files" and not "Text files" when you use Notepad or another editor to create and save the file.
Otherwise Windows may add a hidden .txt file extension and you end up with a not working userChrome.css.txt file.

You can also use the Stylish extension to make it easier.

• Stylish: https://addons.mozilla.org/firefox/addon/stylish/
• http://userstyles.org/help/stylish_firefox