Last year I finished my master’s. I spent the whole year working on experiments for my master's thesis. Part of the results I obtained were published in my master’s thesis, which was submited via an online tool to my university; so there is proof that that work is mine, I guess.

Then I was offered to stay in that research group and do a PhD, but I refused. They went as far as to threaten me with publishing my data without crediting me. I still refused.

Now it has come to my attention that my data has been used to write a paper that will soon be sent for publishing. Besides that threat from months ago, no one has asked for my consent or permission to use my data, and I don’t even know if my name will be among the authors. Even if it does, I won’t be first author, because they will be assigning that to the person who wrote it.
(I’m not sure how much they have written themselves, as I already had written nearly everything myself for when I had to hand it in for it to be graded for my master.)

My questions are: if I am actually asked for permission before they send it to a journal for publishing and they say they will include my name but not as first author, is it worth accepting or should I refuse? In the case they don’t ask me for permission, what should I do? Mail the journal once the paper is published along with my master's thesis and the raw data (all I have are excel files) to prove the data is stolen?

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I understand the confusion in some answers, as my case is very chaotic.
As far as I'm concerned, my thesis is not available to the public. Only my advisor and the director of the master have access to it (maybe more people at the university, but it's definitely not available to the public). 
I also know from a case of a person who did his master's thesis in this same research group and then left, and he was asked for permission before the paper using his data was written. He was also credited as an author. 
Sadly, the fact that the boss holds a grudge against me for not wanting to stay and her having a favorite (the one who will be credited as first author) also plays a major role in this.
In my field, being a first (or last) author means you are who worked the most in that paper. It's usually the boss/advisor and the person who obtained most of the data by working in the lab.