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

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

See http://kb.mozillazine.org/Editing_configuration#How_to_edit_configuration_files

#personal-bookmarks menuitem.bookmark-item,
#menu_BookmarksPopup menu,
#menu_BookmarksPopup menuitem {
 font-size: 12px !important;
 max-width: none !important;
}

Sorry I don't understand - I have just installed Firefox and the userchrome.css file is completely blank - not sure what to do?

In Firefox 3.6.x you can use this to go to the #How_to_find_your_profile|Profile folder. Help > Troubleshooting Information > Open Containing folder - see also Use the Troubleshooting Information page to help fix Firefox issues

C:\Users\<user>\AppData\Roaming\Mozilla\Firefox\Profiles\<profile>\ (AppData is a hidden folder)

In the chrome folder in that profile location you will find the example files userChrome-example.css and rename that file to userChrome.css. You can copy and paste the above posted code at the end of the file.

I have done that but it doesn't make any difference.Do I leave the text exactly as it is in the 'example' file before saving it as userchrome.css and adding the script? Sorry but I'm unsure what I'm doing with this !!

It is better to rename the file. Windows may add a hidden .txt file file extension if you try to save as userChrome.css, especially if the file type selected in the save file window is Text. You may need to place quotes around the name: "userChrome.css"

Sorry - it's not working - also Firefox is not retaining my preferences/settings - back to IE8 I think................