When you open google.com in Firefox, Safari or any other browser than Chrome, you'll sometimes see an ad for Chrome. Usually, Google's promotional messages suggested you should try Chrome because it's fast, but now Google has a new ad: "Google works better with Chrome". Clicking "Yes, get Chrome now" sends you to the regular Chrome homepage, which promotes Chrome as "one browser for your laptop, phone and tablet" since "Chrome brings your open tabs, bookmarks and recent searches from your computer to your phone or tablet, and vice versa".
Chrome has many features that integrate Google services (translation, spell checking, reverse image search, Safe Browsing, Cloud Print) and many Google features only work in Chrome (voice search, offline Google Drive, Google Play Music uploading).
Maybe Google should link to a page that explains why "Google works better with Chrome", since it's not exactly obvious. A Twitter user says that "if you can't get a 1 field 2 button form & list of results working cross-browser, you're doing it wrong", while another one finds that "'@google works better with Chrome' sounds like a bug report. Can you just fix it please?". F. Nonnenmacher thinks that "Microsoft was sued for less than that".