The Senate literally makes it's own rules, and chooses how to organize itself. The rules are so convoluted that the Senate established its own office, the Parliamentarian, to advise on those rules. This is a non-partisan office that serves the Senate as a whole. They get no actual vote, but their decisions affects how work in the Senate progresses. (In particular, they are the ones who make decisions on these "reconciliation" bills that provide a way around the filibuster.

Trump can't fire her, because she works for the Senate Leadership. The Majority Leader can fire her, but won't (at least so far), because once the rules start breaking down, it's impossible to get anything done in the Senate. Republicans could just dispense with the rules to give the majority more power, but they are terrified of what Democrats would do with that power if they ever gained the majority afterwards.

So, if we ever do get to the point that Republicans fire the Parliamentarian, or further restrict the filibuster, it will be because Republicans are counting on never having to give up control of the Senate again.

Yes, they resisted for now. But respectfully, I think believing in some magical power of the parliamentarian epitomizes institutional thinking, and ignores asymmetric problems in fighting fascism. A democratic institution has to resist every fascist attempt at takeover to survive; a fascist attempt to take over only has to succeed once. And the parliamentarian is an incredibly weak link in our institutional chain, while other stronger links like an independent DOJ and Supreme Court, have already fallen.

Thune didn't fire her yet because he is an old-school Republican trying to maintain traditional GOP power structures. But think back: in 2020, even the House was not overwhelmingly MAGA, with traditionalists holding power, and the Senate was barely Ted Cruz. By 2024, the House was overwhelmingly MAGA, and the Senate had plenty of MAGA. The Senate GOP now thinks they retain independence but fears the bad polls, and shows how afraid they are by still hiding behind Thune who is the lighting rod for Trump. And Trump will keep pushing, endorsing opponents, and sic-ing MAGA's base on resisting senators. He's a narcissistic sociopath and will never accept, "no."

Eventually Trump will push hard enough and the Senate will be weak enough to elect a Mike Johnson figure who is as completely broken and spineless. We have to hold out until 2028 before the GOP loses the Senate, most likely. Then they will do what Trump does via his other captured institutions - break the law and let others fight about it in court while he wields and consolidates power. A parliamentarian isn't even a speedbump at that point.