Even if you have FireFox as your default browser, IE9 libraries are still present on your system (windows 8 has IE10 libraries of course).
If the trainers were working before now they aren't, you will need to think of what you have installed since that gap of time that could be affecting the trainers.
As a test put all your trainers in a folder on c:\ called trainers if you would like, and then set the folder to be excluded from your AV scans (can do this is in MSE settings, you can do it with the anti-spyware settings also). Make sure to clear out the scan cache on MSE and your Anti-Spyware apps.
Then re-check, if it works then your security software is blocking them in some way.
[Edited by DABhand, 3/24/2013 8:05:30 PM]
The trainers are already in an excluded directory. There's nothing running now that wasn't already running a few months ago. I run CCleaner regularly, which clears out those caches.
It doesn't look like a trainer being compatible with Windows 8 is an issue, since the trainers for Red Faction: Guerrilla, Just Cause 2, STALKER: CoP, and The Witcher all work fine.