Let them modify the browser in any way they want. Let them write plugins that can do anything. But inform users who install a third party plugin what that plugin can do. Like "This plugin wants the right to read all data on all websites you visit. In theory it could capture your bank login and transfer it to some other server".
Another bad decision by Firefox to limit users ability to do what they want is that they don't support the File Access API:
https://web.dev/file-system-access/
This gives Chrome users the ability to build simple web pages that can be used like native applications.
An empty html page with a textarea, load and save buttons and you have a text editor!
I hope Firefox will notice that they lose market share because of restricting users freedom. And turn the ship around.