If that is unacceptable in general, then why don't you check the settings of apps before you give them permission to access the internet?

Mozilla is not even sneaky about it. On the Google Play download page, the privacy notice for the app tells you that optional usage tracking is on by dafault. https://support.mozilla.org/en-US/kb/focus#w_privacy

Perhaps you should read the privacy notices for apps if you are concerned? It seems perfectly acceptable to me.