firefoxPref script preferences being ignored

I’ve been trying to test the differences between HTTP 1.1 and HTTP 2.0 but have been unable to get the firefoxPref script option to work.

I’ve been using the following script

an example of the test results are available here:

Inspecting the requests show the H2 pseudo-header fields

and the User-Agent has also not been changed.

Am I missing something?