The or operator is left associative, and since there is another
argument after the first term, the interpreter tries to apply whatever
the or-expression evaluates to. If the first operand is unset, it
evaluates to removeAttrs, and everything is fine, but if it is set to
a boolean (which is what it should be set to), then it tries to apply
a boolean to arguments, and we get a type error. Bracketing
explicitly with parentheses fixes this.
It presumably went unnoticed because not many people have tried
setting the option
`programs.firefox.profiles.<profile>.search.engines.<engine>.isAppProvided`.