Well, that worked for me, too (sorry that I didn't mention that). A problem occurs in the app I am testing, and is even stranger than I thought. I have the app working in two places: on a testing server
in the cloud on someone else's computer and locally on my laptop. The test doesn't work with the former and works with the latter.
I have Firefox 58.0.1 and TestCafe 0.18.6.
I am not currently able to pinpoint the problem/build a minimal example. If I succeed at this, I'll get back here.