Garbage. It's not "nearly impossible to fire a teacher." We can see cases left and right where teachers are fired very rapidly. What tenure guarantees teachers is due process: you cannot be fired without good cause. The only way an incompetent teacher gets tenure in the first place is if the principal & superintendent of the schools completely drops the ball.
Teachers don't mind being evaluated on student performance. What they want is a reasonable & fair way to evaluate this. There was a case in NYC of a teacher evaluated on her performance. She did incredibly poorly - one of the worst math teachers in NY based on their rubric. Her 8th graders had previously been in the 99th percentile. Following the 8th grade specific test, the students dropped to the 89th percentile. That was one of the biggest class drops. Was she ineffective? Not by any stretch of the imagination. She did not "teach to the test" for the state 8th grade test. The students did quite well on that test, though they missed some minutiae that in the grand scheme of things are irrelevant toward further success in mathematics. What her 8th graders did was prepare for the 9th grade exam (Integrated Algebra) - they were advanced. A significant percentage of her class aced the exam. I don't personally know any teachers who get more than a couple percent of their students to get a perfect score on that exam. Her class was among the very best in the entire state on the 9th grade exam, but her performance was poor, because the kids did poorly on the 8th grade exam.
You might be thinking, "yeah, but what idiot wouldn't *realize* that her students were advanced and did so well on the 9th grade exam." The problem is, these rules have to be written down. It's pretty hard to craft a policy that has to be followed to the letter, yet also completely applies common sense.
Think about this for a second. If a teacher knows they are being strictly evaluated based on their students' performance, and ranked against other teachers, what do you think they're going to do? They're going to teach to the test.
So, here, we have you, the public, saying that teachers need to do a better job, they need to do more than just teach to the test. And at the same time, rig the system so that any teacher who goes outside teaching to the test is penalized. The teacher's union has thought this through; hence the objections.