Maybe, for a few reasons. First, your restatement of Myers is incorrect. Myers does not state that the President can fire any federal official. Myers states that the President has unfettered discretion in removing executive officials. This is a crucial distinction because there are, of course, other federal officials—those in the judiciary and the legislature. The Constitution gives zero authority to the President to directly remove these officials (recall separation of powers).
But there's another subset of federal officials we're forgetting—those in the independent agencies (e.g., the FEC, FTC, Federal Reserve, etc.). These agencies don't really fall under the executive, or any branch for that matter, hence "independent." They're sort of quasi-legislative quasi-executive, and for some, quasi-judicial.
Humphrey's Executor gave congress the power to condition firings of officials in these agencies (e.g., you can only fire them if they commit negligence, through special procedures, etc.). These conditions essentially limit the President's power to remove ("fire") federal officials that might appear to be under the executive.
Then Morrison came along and held, congress can condition those firings, but only if it does not "unduly impede" or "trammel" the President's removal power. More recently, the court decided Seila Law, which made even the further distinction between officers heading single-member independent agencies and multimember independent agencies (the former being subject to the President's removal power).
In any event, a lot of the agencies you hear about are actually independent agencies: CIA, SEC, CFPB, EPA, FCC, FDIC, NASA, and, since 1971, the USPS, which is headed by the Postmaster General. The USPS is itself sort of a multimember agency, since it has a board of governors, but it is run by and large by the Postmaster General, so it shares some characteristics of a single-member independent agency. Thus, the answer to your question is a little less clear with the case law currently. However, given the composition of SCOTUS today, and Justice Roberts on the bench (a supporter of Justice Scalia's unitary executive theory, which—oversimplifying here—says that the President has the sole authority to fire those who wield executive power), it would not be surprising if the current Court finds that the President can fire the Postmaster General. Of course, this is all conjecture. This is all a long winded way of saying: maybe.