FF5 workaround for workaround Rated 4 out of 5 stars
I had the same problem as critStock where I couldn't add, edit, delete, import, or export any shortcuts. To get around this, I found the shortcuts.xml in my firefox profile (through a file manager - finder on mac, or explorer on windows) and was able to edit the entries in there.
Each shortcut has a key, title, url, groupId, iconData, iconDataSmall, iconDataLarge, and iconSource that can be defined. Copy&paste an existing shortcut and edit as needed.