The Favicon Picker Firefox extension assigns an image of your choosing to represent sites you bookmark - and provides a great way to free up real estate on the bookmarks toolbar and visually represent your favorites.
Reader Jason says:
I love the bookmarks toolbar... the only problem is I could never fit as many bookmarks as I'd like to. Something I discovered this past week that is unique to Firefox is that you don't need to have a name for bookmarks in the toolbar, the favicons (or lack thereof) will suffice. This is really enhanced by the Favicon Picker extension for those websites that don't have favicons. You can set your own. Now I have one-click access to over two dozen sites.
Click on the image above for a larger view of Jason's colorful icon-studded bookmark toolbar. Thanks, Jason!
Update: Here's more info on how to get this to work. Once you've installed Favicon Picker and restarted Firefox, right-click on a favorite on your toolbar and if it doesn't have an icon, choose any image on your computer and assign one. Then, to view only the icon and not the text, remove the text from the "Name" field, leave it blank and hit OK.
Favicon Picker 0.2.5 [The Extensions Mirror]