What is being send to/received from "safebrowsing.google.com" when I open Firefox?

You are getting an up to date list of blacklisted URLs that are known to contain phishing and malware. The update happens shortly after startup and once again every 30-45 minutes.

Mozilla claims the following:

"No information about you or the sites you visit is communicated during list updates... in the event that you encounter a reported phishing or malware site [b]efore blocking the site, Firefox will request a double-check to ensure that the reported site has not been removed from the list since your last update."

The accepted answer here is unfortunately completely wrong, as this feature can absolutely be disabled easily.

Despite what it says, none of the messing with the hosts file is necessary, you can just change the "Block reported attack sites" and "Block reported web forgeries" settings in Firefox's Security Preferences and this will stop both the updates and checking the lists from happening.

If you manually fiddle in the about:config settings, you must also flip this preference: "browser.safebrowsing.downloads.enabled" which disables updating of blacklisted downloads as well.

Mozilla claims that no information about search queries is ever sent, only the double-check of an encountered reported phishing or malware site, as mentioned above.