Question about adding co-first author
Sadly enough it is pretty common. Not universal, but more common in certain fields than others. You can fight it, but I recommend that you don't. 

The reasons for letting it go is that you have finished and have a position (I assume). Over the course of your career, this sharing of first authorship on one paper will mean little, even if it is based on your dissertation. But the good will of your advisor, and possibly the other student can pay dividends in the future. 

Your dissertation work is your first work, but, hopefully, not your best ever. And having the old professor being willing to work with you to advance your career and so breaking ties now is probably a sub-optimal move. 

Yes, fairly normal. Questionable ethics. But optimize a path for yourself, and a single paper is only a small part of it. Yes, I know it is disheartening. But the long view is worth considering. 

And, I've assumed you are A, here, hence the wording.