The fix is actually for another Firefox plugin: Colorzilla. I was having some problems making the HTML Validator work back then and decided to install Colorzilla first. It also had some problems but after some Googling, I found a fix that worked. The latest Firefox binaries needed to be downloaded and its libxpcom shared objects (libxpcom*.so) overwrite the current ones found in /usr/lib/firefox.

Feeling that I couldn’t do much without the HTML Validator plugin, I gave it one more try. The thing worked!

After upgrading to Ubuntu Gutsy Gibbon, some of my Firefox plugins went crazy (HTML Validator, Colorzilla and NoScript). Then I remembered having the same problems and fixed it again.

Just wanted to post that- might be able to help someone.