The method used in the gif is a standard start up, in which case Normandy specifically waits for the UI to be ready before making any changes. That is expected not to work.
The way to make this work is either by launching from the installer or by passing
--first-startup to Firefox on the command line.
The thing that worries me is that according to comment 0, you already tried using the installer to launch Firefox. Can you go into more detail about what you did in that case?
We've also used attribution code and the stub installer method of starting up Firefox for the first time and the issue is reproducible in this way as well. Normandy is either too slow in enrolling or the page is too fast when it is displayed.
Assuming that you were using recipe 931 (public link), this is definitely worrying, since I don't see any thing that should have not worked there.
Additionally, it looks like the study did enroll correctly. That makes me wonder if the pref is being read as I understand it, and if the
trailhead.firstrun.didSeeAboutWelcome acts as the recipe is expecting. If you set the pref values used in the recipe as default prefs in
prefs.js, does that work?