Testing script failed with code: 126

I just wanted to do a Test with BackstopJS and ran into an error that my firefox version is too new basically. I downloaded an old version and changed the path for slimerjs to the old firefox version.

Now i get this error: "Testing script failed with code: 126"

Can anybody tell me where i went wrong or what this error means?

No file, test, or entry point is named. Start by reproducing the BackstopJS run with the changed SlimerJS and Firefox path, then inspect the testing script output around code 126. Done would mean identifying the failing executable or configuration and documenting a reproducible fix.

Written by the indexing model from the issue text.