With people shifting their layouts over to Flexbox, relying on "box-sizing: border-box", "display: table-xxx" and friends, older browsers will mess up the layout beyond belief. IIRC some older browsers even failed to apply an entire CSS file if they couldn't parse a single rule - good luck with ::before, ::after, ::nth-child and friends.
CSS, unfortunately, isn't really backwards compatible unless you invest a LOT of effort into testing and development.
Good riddance. Those older browsers are also going to be full of unpatched security holes, and if websites don't work people will eventually quit using them.
Many recently updated websites of unicorns/startups look very bad or often completely unreadable on older browsers e.g. IE 9. Can they live without enterprise sales?
In Firefox you don't need to use an old version; just do it from about:config: media.autoplay.enable = false.
Just add a bookmark to kill iframes, then you just click the button to instantly remove any iframe adverts (which includes all of google's ads).
Why? This still allows the ad tracking and other malicious Javascript, and it still requires you to recognize that there are ads on the page. The cost in mental effort is significant; if you think it isn't, live completely ad free for a month or two to reset your tolerance.
Ad tracking uses cookies, not javascript, and isn't "malicious" as such. If you're worried about that, just disable cookies and/or javascript.
It depends on the tracking. They are far more advanced than simple cookies these days. Some do use javascript to load things and/or report back without resorting to easily-deletable cookies. Whether something qualifies as malicious or not is a matter of opinion. Imho any tracking attempt by unnamed third parties (ie without my say so) is evil.
For example, Chrome keeps Google analytics connections open for 4 minutes since the last request, even cross sites. This is a cookie less, IP less of uniquely identifying a user.
That is for analytics, not adverts. Google adverts only uses cookies to track users AFAIK.