You can increase the String pref layout.css.devPixelsPerPx from 1.0 to 2.0 in 0.1 steps to see what works best.

In Firefox 3.6 and later this pref is a String value parsed to a float and allows to fine tune the dimensions of all elements more precisely.

To open the about:config page, type about:config in the location (address) bar and press the "Enter" key, just like you type the url of a website to open a website.
If you see a warning then you can confirm that you want to access that page.

Thanks a lot for your reply. Unfortunately it does not apply to my query. This configuration parameter seems to only affect the height of the tabs in the tab-bar. It does not change the font size. What I am looking for is the name of the parameter that determines the size of the type fonts in that part of the Firefox window that is not part of the WEB-page, for example the text in the menu bar, the address bar, the bookmark bar, the tabs and the status bar. Is there anyone who can tell me how to find that information?

Thanks again,

Arie.