What if Trump says he has fired SCOTUS Justices who voted against him?

Many. And yet he remains free and in the US. If Trump is currently unable to make his arbitrary pronouncements stick on a random Salvadorian dude, I can’t see them working on the most powerful and protected individuals in the US system of government in the immediate.

Yup but if the 1/6 rioters had targeted the supreme court building I bet they would have suddenly started caring very much about the rule of law and pivoted to a very strong line on the enforcement of insurrection statues.

I wonder if he could suspend funding for the SCOTUS?

I’ll bet at some point in the process, he probably has the power to do so but, again, not the authority. Congress appropriates funds and after that, someone in the Dept. of the Treasury has to move the funds from the Treasury to the Supreme Court. If you ascribe to the unitary executive theory, the executive could order the Secretary of the Treasury to withhold the appropriated funds from the court. Sure, he’d be violating the appropriation bill and the Impoundment Control Act. (I think; the Impoundment Control Act mostly governs executive branch spending but I’m guessing without really doing the research that the disbursal to the court of appropriated funds would be subject to it too).

But note that ceasing to pay them still doesn’t stop the court from issuing orders. Again, it might encourage them to quit (though most have perfectly livable independent incomes) but it’s still not the same as firing them. If anything, it would put them at the center of a political dispute so historic that their book advance fees would go into the multi-millions. Even their clerks would probably work for free. Most of them expecting mid-six figure law firm bonuses when they leave. Their clerk salaries are essentially meaningless in comparison.

Maybe they would have briefly but they would have quickly forgotten about it just like most of the GOP/MAGA types in Congress. They were outraged for a minute but within a week or two not so much anymore.