From the First Posts Review instructions:
I'm Done is only available if you've done one of the following:
- Vote up or down to rate the question's quality and usefulness
- Edit to improve the question's appearance or clarity
- Comment to leave constructive feedback for the author
- Flag to notify the moderators of serious problems
- Close questions that cannot or should not be answered here
When in doubt, upvote the question and leave a comment to help the user out, or click Not Sure if you are unsure and want to skip this question.
That means that you have to do one of the above actions to click on I'm Done button. And if we click on I'm Done, we get new post to review and our review count is increased by 1 (I think that's not important for anyone).
But I found (accidentally) a minor fault in First Post Review.
What I did is: I got a question, where I commented. And I'm Done button got enabled. But deleted that comment as I found the comment was not useful. So I expected I'm Done button to be disabled. But it was not disabled. It remained enabled. (That means it was enabled without reviewing.).
So I suggest to disable the I'm Done button again when comment is deleted. (Same as when we vote the post then the I'm Done button is enable and when we rollback our vote(unvote) then I'm Done button get disabled.)