From Jonsdocswiki

HTML Validator is a Firefox addon that validates websites as you view them. This is a very useful tool for website developers.

Troubleshooting

I get a HTML Cache is Empty error all the time

This error is presented all the time Firefox's cache is disabled. Re-enable the cache either using about:config or by using whatever mechanism you used to disable it in the first instance.

See also

• Author's website
• Author's Mozilla page
• HTML Validator User manual