Change default programs in Firefox's system handler?

[connorhedgehog]

when i log into roblox studio (or any program that uses website auth), firefox has a default program it sends whatever the link is to, seen here:

problem is, when you first find a program, you can set it as “always use this”. i did this for the wrong program, and i can NOT find out how to change it. it’s clearly a kde thing, being the system and all, but i can’t find where or how anywhere on my computer or online. what do i do?

[Ofnuts]

In Firefox:

And if you pick “Use system default application”, see Plasma’s Settings > File association, edit the file type, and move the required application to the top of the list.