You can ignore (CSS) errors in the Tools > Error Console. Those errors are only useful if you develop websites and want to check the HTML/CSS and JavaScript code for errors. Most errors are code for other browsers like IE or browsers on other platforms or just typos or the result of bad coding.

What is the problem?

See Websites look wrong or appear differently than they should

Ignoring the error doesn't solve the problem. I'm getting the same error also, on code that VALIDATES per the w3c CSS validator.

Firefox is wrong, it's seeing an error when there is none.

This is FF being pedantic about code...

If you're trying to set a CSS Style measurement property with Javascript, you need to specify the measurement unit (eg "px", "%", "em", etc as you do in CSS) as a string. Passing an integer (without the measurement unit) into the property value causes the error.

Example:

var w = 10;    //Assign an integer to variable w

/* Now let's try passing w to set the CSS Width property of element */

element.style.width = w;       // This Generates an error in FF, 
                               // but works fine in IE (IE doesn't generate 
                               // an error because it just assumes you mean "pixels" 
                               // when you pass an integer in the script.)
element.style.width = w+"px";  // This works in both FF and IE. Will set the width to 10 pixels.