There are plenty of lists around the place, top 10 this 10 best of that. Here is a twist in more ways than one.
Firefox is a highly customisable browser, as such they don’t make it hard to create extensions for it. If you subscribe to the RSS feed for new Firefox extensions there must be at least 10 a week. Generally extensions at some great functionality to Firefox. Just for your information they are not like Internet Explorer Add-ons that you have to remove with the Add/Remove Programs. Firefox extensions are very easy to remove inside the browser itself and when they are gone they are gone. So given all that there are some duds.
Here is a list of the Worst 100 Firefox extensions. Ironically they can be found on the Unencyclopedia. To add further to the anti of everything the Worst Firefox extensions list links to the 100 Worst Lists of all Time.
Firefox is not perfect all the time, just most of it.