I get the error message:
There was an error downloading .
I am using Lucid Lynx and Firefox 11.
Two of the add-ons in question are Easy Youtube dowloader and Flas Video downloader.
I remember that I also had this problem on Windows when I was still using Firefox. My solution back then was removing my profile and making a new one.
In Ubuntu you can access your profiles by running the following command at the prompt: