Firefox addon or other tool to locally validate HTML pages

Question

I have a rails project with a lot of duplicate id's (it looks like id's were used like class should have been used), so obviously this is bad.

I need to refactor everything to get rid of the duplicate id's, and need some addon for Firefox (ideally) to validate the page as I view it to ensure there are no duplicate id's, since it's not something I want to chalk up to "look at each page by hand".

Is there an addon or other tool that will allow me to do this? (I have checked through the firefox addons, and found nothing that really works, they all just redirect to the w3 validator, which I can't use, since the app is local).

EDIT: I should have mentioned, I want this on Linux. HTML Validator does not work on Linux

Answer 1

The Web Developer extension should allow you to validate local HTML.

Right-click -> Web Developer -> Tools -> Validate Local HTML (Ctrl + Shift + A)

Answer 2

Install the Web Developer Toolbar, restart your browser and press control+shift+a

Answer 3

Or use HTML Validator, it can even clean your code up using WWWC's Tidy.

Answer 4

Total Validator works well, too.