Limitation of free plan in firebase
Solution 1:
That means that you can have only 50 active users at once, only 5GB data to be transferred within one month and store only 100 MB of your data.
i.g. you have an online web store: only 50 users can be there at once, only 100 mbytes of data (title, price, image of item) can be stored in DB and only 5 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 mbyte size and users will be able to attend that page only 50 000 times).
UPD: to verify the size of certain page (to define if 5gb is enough for you) - using google chrome right click anywhere on page - "Inspect Element" and switch to tab "Network". Then refresh the page. In bottom status bar you will amount of transferred data (attached size of current stackoverflow page, which is 25 kbytes)
Solution 2:
From the same page where the question was copied/pasted:
What is a concurrent connection?
A connection is a measure of the number of users that are using your app or site simultaneously. It's any open network connection to our servers. This isn't the same as the total number of visitors to your site or the total number of users of your app. In our experience, 1 concurrent corresponds to roughly 1,400 monthly visits.
Our Hacker Plan has a hard limit on the number of connections. All of the paid Firebases, however, are “burstable”, which means there are no hard caps on usage. REST API requests don't count towards your connection limits.
Data transfer refers to the amount of bytes sent back and forth between the client and server. This includes all data sent via listeners--e.g. on('child_added'...)--and read/write ops. This does not include hosted assets like CSS, HTML, and JavaScript files uploaded with firebase deploy
Data storage refers to the amount of persistent data that can live in the database. This also does not include hosted assets like CSS, HTML, and JavaScript files uploaded with firebase deploy
Solution 3:
These limits mentioned and discussed in the answers, are per project
The number of free projects is not documented. Since this is an abuse vector, the number of free projects is based on some super secret sauce--i.e. your reputation with Cloud. Somewhere in the range of 5-10 seems to be the norm.
Note also that deleted projects take around a week to be deleted and they continue to count against your quota for that time frame. Ref