Firebase made the pricing changes in August 2015 (I was a customer at the time and saw our bill increase). They wrote to customers about it, and published a blog post about it. For some reason you thought $25 per month was reasonable for a service with thousands of users and didn’t question why it didn’t apply to you?
The problem here is that for whatever reason Firebase weren’t measuring your traffic correctly, and rectified that problem in March/April of this year. Yes their customer service is shitty, and yes they should have been measuring it correctly when they changed the plan so you had more warning. These are all due to Firebase’s poor customer service that has led to your problem, but it’s still your problem. You need to own this. Writing medium post’s bitching about a shitty service provider instead of fixing it doesn’t help you or your customers. There are shitty service providers everywhere, you accept the risk or you don’t. Being blind to it is on you.
Wilful ignorance thinking $25 a month is ok for the service you’re getting that is critical to your business is on you.
Where was your logging of errors in your app? Where was your third party monitoring of the service? There are a hundred things you could have done to prevent this, and all you did was write medium post.