Even I uninstalled the Wizz RSS extension, “Wizz RSS” folder was still existing in Firefox 3 bookmark. Then I found this forum which explains how to remove the folder.
To inform, I am using portable Firefox. And besides that, I always make my Firefox profile corrupted because simply changing the data in the file using a text editor. Therefore, make sure you backup the Firefox profile, so that can be recovered later. Otherwise you need to install all the extensions again.
Make sure you delete all the bookmarks within “Wizz RSS” folder.
1) Install SQLite Manager extension.
2) Backup “places.sqlite” file in the Firefox profile, so that you can recover if anything goes wrong. Backup when the Firefox is closed.
3) Run SQLite Manager, and open “places.sqlite”
4) Open “moz_bookmarks”
5) Go to “Browse & Search” tab
6) Search for the title “Wizz RSS” (I only found one item)
7) Delete it.