NTHSA has a large database of rollover risk estimates, at least for base model cars - here's a whitepaper on the subject. The static stability factor (SSF) rating is a simple half the track width divided by CG and is the only characteristic of the car used to assess rollover risk in single and multiple vehicle accidents when combined with other risk factors unrelated to car geometry (road conditions, age of vehicle, if the driver is male or lives in Florida or other high risk state...
)
https://crashstats.nhtsa.dot.gov/Api/Public/ViewPublication/812444
The base Fiesta is in the database with an SSF of 1.29, which is on the low end of passenger cars as a whole, but is basically on par with most other narrow wheelbase compact cars and hatchbacks. Comparable cars are Toyota Yaris/Matrix (1.27), Fiat 500 (1.28), Ford MkII Focus Hatch (1.30), Honda Fit (1.32), Nissan Versa (1.28), and all are better than 95% of SUVs and Trucks with the exception of those that are on the lower side with very wide track width.
Incidentally, with a track width of 57.7", that means the base Fiesta has a CG height 22.4" inches off the ground. The ST's CG is about 0.4" lower with the same front track width, so that means the ST has an SSF of 1.31 and CG height around 22.0" at factory ride height, which might be useful info for anyone who cares about such things., especially once you know that lowering the car more than about 0.5" past the stock ride height starts to absolutely murder the front suspension geometry and roll center height
None of the other cars listed above have ever been banned from autocross because of rollover risk, and people run them all the time, which leads me to believe the hubbub about the accident in question is way overblown, and the likely cause was the factors mentioned before - the combination of bad setup from compromised suspension geometry due to lowering plus initially soft Eibach progressive springs and tall tires and a big helping of driver error, unfortunate road surface and freak circumstance.