Firebase has three plans, one of them is free called Spark Plan. Using this free plan you can have only 100 active users at once, only 10GB data to be transferred within one month and store only 1 GB of your data. i.g. you have an online web store: only 100 users can be there at once, only 1 GB of data (title, price, image of item) can be stored in DB and only 10 GB of transfer - means that your web site will be available to deliver to users only 5gb of data (i.e. your page is 1 MB size and users will be able to attend that page only 10 000 times).
And providing 10 Virtual Device Tests per day and also 5 Physical Device Tests per day.
Other two paid planes are -
- Flame Plan - $25/month
- Blaze Plan - Pay as you go
You can check those planes from here.