How can I add a favicon to a bookmarklet in Firefox?

You can try this trick:

1. Find a normal link (without JavaScript) for the website with the correct favicon.

2. Add that link to your bookmarks toolbar as a normal bookmark.

3. Finally, change the properties of the bookmark you just added and change the location to be the actual JavaScript bookmarklet you want (you want the location from the bookmarklet that doesn't have the favicon).

Now you've turned the bookmark from step 2 into a bookmarklet with the desired favicon and JavaScript.

The Favicon Picker 2 extension will let you do this.

How to Use: Just right-click any bookmark and select "Properties,"
then click "Browse..." to select an icon or paste a URL to an icon.

Also, you can uninstall the extension when you're done and the icons will remain.