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.

Clear the cache and the cookies from sites that cause problems.

• "Clear the Cache": Firefox > Preferences > Advanced > Network > Offline Storage (Cache): "Clear Now"
• "Remove the Cookies" from sites causing problems: Firefox > Preferences > Privacy > Cookies: "Show Cookies"

Thank you but the problem persists

Well this was happening to me when my firefox loaded a website with a third party skin that installed by another coworker.

The reason was that the line-height value was written with backslashes.

It looked like this:

 /* IE 8 Hack */ line-height: \12\ /; 

I think it was a mistake caused by some upload or another, or maybe there was a comment there which was removed previously. The only reason I post the problem is to show that you may have to carefully check the value.

As the error mentions "Error in parsing value"

It does not mean the FF had any problem with the property (line-height), which is what I first concluded.