I may also suggest leveraging auditd to track what processes are attempting to establish network connections if you want to specifically pin down the source of all your closed connections. Note that this will eat up a lot of log space quickly if you use it for more than a reasonable span of time. At the very least, this will help you determine what network activity is part of the expected baseline if hardening and monitoring is your goal.

Thanks for the provided material and info, Jonathan;

Unfortunately I can not prevent Firefox entirely from connecting to apparently random web addresses once I open it; not even with the settings (malware, phishing) you have recommended me to try; not even with disabling all addons. Three servers that were contacted while/upon an about:blank invocation: