She is probably looking for the limits or boundaries as they apply to herself, as well as trying to learn how things work, and as a byproduct of that is inadvertently testing your emotions.

How did you respond to her? A response in which you up the ante and engage her on an emotional level may reinforce her to initiate power struggles: You may teach her that's how things work.

One possible response to her may be to acknowledge that she is unhappy or displeased with XYZ situation, while helping her to place XYZ situation in perspective in the grand scheme of things (kindly helping her see that XYZ disappointment is not such a big deal). This could be done in the moment or brought up later, in an understanding manner, when things have calmed down.

You may wish to explain disinterestedly and nonchalantly that firing a mom is not how things work.

In attempt to get her working WITH you (no longer acting in opposition to you), you may wish to
1) say wistfully "That's an interesting concept..."
2) ask if she'd like to write a book about it (how things would be different for an imaginary girl after she fired her mom),
3) ask if she'd like to learn more about how things work: firing, human resources and the laws, rules, procedures, and documentation which must be followed.

This could be an opportunity to explain that everyone has rules and/or a boss above them, which dictate the rather narrow parameters within which they must operate.

Following along these lines, this could also be an opportunity to talk to her about self-advocacy and help her learn how things work. You could establish a process for articulating her grievances within the family, a process which she could draw on in her future life situations. For example, you might have a family meeting. She might be allowed three sentences or be allowed to speak while an egg timer runs (channeling her energy into focusing on formulating her best case). Then it is your turn to speak: limited by three sentences, or an egg timer. Etc.

Who knows, she may be a lawyer someday, furthering justice and righting many wrongs.

Wishing you all the best.