I am not sure that is possible. What is possible (Certainly in Windows) is

• change firefox preferences that change all the menu item etc sizes
  

(That can introduce glitches in the tooltips on bookmarks sidebars bug 644952 )

• change OS options that again change display
• open the library as a tab and zoom in to magnify it.
  edit - attached image

I am not sure if any of those options would help you.

Which bookmark font do you mean?

The bookmarks menu of the menu bar or elsewhere like the Bookmarks Toolbar or Bookmarks sidebar?

I am referring to the Bookmark sidebar where people typically have a long list of website links in a extremely small font size. It is a chore to read as you scroll down the list trying to find a particular site. I find it difficult to believe that I can't make this larger without increasing the size of everything.

Try this code in userChrome.css in the chrome folder below the @namespace line.

• http://kb.mozillazine.org/userChrome.css
• http://kb.mozillazine.org/Editing_configuration
• Help > Troubleshooting Information > Profile Directory: Open Containing Folder

You can use the ChromEdit Plus or Stylish extension to have easier access to the customization files.

• ChromEdit Plus: http://webdesigns.ms11.net/chromeditp.html

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

#sidebar-box *,#bookmarksPanel *,#bookmarks-view *,#history-panel *, #historyTree { font-size:12pt!important; }