Well, Chrome recently released their 92nd version... The same thing could happen to them really soon. Should they keep the user agent as version 99 indefinitely too? User agents shouldn't be as relevant as they are.
When a new Firefox release breaks a website, the browser is broken.
When a new Chrome or Safari release breaks a website, the website is broken.
I hate myself for typing that out, but it's true. Chrome has the luxury of dictating terms to sites that IE used to have 10 years ago. Firefox has never had such a dominant position, so they need to shoulder some of the burden themselves to ensure remaining users aren't forced to switch.