Ahh I'm so sorry to hear that happened to you!
Was it just one person's decision?

I don't think it's fair, at all, for them to be thinking "she's not courteus enough" and to not bring it up with you, give you advice on how to do the job better. You were new and learning the ropes, how can they even expect you to be perfect at it when you've only just started? If you weren't quite 100% yet then that's soo normal and you can learn, but you need them to HELP you... and jeez, they really didn't do that!

You know, though, it seems like they have thrown this word at you without really seeing any evidence of you not being courteous, it sounds like an excuse. I was once told I was "blunt" in an assessment for a new job I had, and that I ignored the kids when they asked me to go to the toilet... and it was an absolute lie (and yet I still took it horribly personally even though I KNEW it wasn't true). I never knew why, other than that woman just took a dislike to me (she was quite mean and nor did I like her back, I was quite intimidated by her as I was only 16)

Perhaps it's something about the person who fired you. You said you ARE courteous so they basically made it up so it must be something about THEM that isn't quite 'right', perhaps jealousy, or feeling threatened. I don't know... but I'm sorry you had to suffer because of their mis-judgement.

I hope you're able to move on from this and not take it too personally. I'm sure you will find a better job with other co-workers who aren't spiteful like this person seems to be.