Slightly tangential, but on the topic of browser extensions: the Chrome extension API served as the basis for Mozilla's somewhat recent Web Extension API [0]. Many, if not most, Chrome extensions can run in Firefox out of the box right now (there's even a Firefox extension that lets you install these from the Chrome Store [1]). This has been an incredible development for anybody doing cross browser extension development (which has otherwise been a fair bit more complicated than simply including "CSS prefixes besides -webkit").
Having different browsers compete on things like this and then having the better technology become standardized and implemented in multiple browsers seems like a fairly positive outcome.
[0] https://wiki.mozilla.org/WebExtensions
[1] https://addons.mozilla.org/en-US/firefox/addon/chrome-store-...