Firefox did a lot of work for web interoperability. It was the first non-IE browser to have gained significant market share, which subsequently forced web developers to write standard compliant code, and test their sites on non-IE browsers, which in turn popularized other standard compliant browsers.
In a way, it standardized the web by breaking out of the chicken and egg loop (people use IE because websites only work in IE because people use IE...).
Nowadays we can use whatever browser we want and 99% of websites will just work. It was far from the case before the Firefox era. I know because I was already using Linux back then, and the lack of IE was very inconvenient.
I have a lot of respect for what they have done.