> Do you believe she was let go because she was black or female?
Possibly. I do think it is likely that she isn't fired if she was a white male. In my experience, people have more patience with people who are in their "in-group". This goes beyond race/gender -- just look at sports fandom for an example. Isn't it odd that players for the team you cheer for can do no wrong, but as soon as they're traded then they start doing a bunch of bad stuff, even the stuff they did while on your team actually now looks kind of bad. In the tech industry role/team/company/race/gender are probably the biggest in-group delineations.
> Do you believe that corporations generally accept ultimatums?
Yes they do. The most common ultimatum is given before employment, where candidates will often give a minimum salary/benefits they will take for the job. I've seen and even been involved in these negotiations where a candidate states that they will not go below this offer, period. And then we counter with a number below that but provide some other incentive we think may counter it.
There are other ultimatums I've seen in the workplace. For example, "I can't work with XYZ or I'm leaving the team". I don't think I've ever seen the response come back as, "good luck finding a new team". It's always been to dive deeper into the relationship.
In fact I issued an ultimatum many years ago where I said I couldn't travel any more due to a personal issue or I'd have to find new employment. I guess you could say they yielded to my threat. I like to think that I was just being honest and they helped find a way that I could still help the team, especially since I only traveled a couple of times per year.
> Do you believe the message she sent out would never be a fireable offense?
I can imagine it being listed as a firing offense in some handbook. I can also imagine that others who have written damning emails have gotten away with no more than a slap on the wrist (or less).