firefoxPref script preferences being ignored

[tpartington]

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

Quote:firefoxPref network.http.spdy.enabled false
firefoxPref network.http.spdy.enabled.http2 false
firefoxPref general.useragent.override "Mozilla/5.0 (Windows NT 6.1; WOW64; rv:45.0) Gecko/20100101 Firefox/45.0 NoSpdy NoHttp2"
navigate https://http2.akamai.com

an example of the test results are available here: http://www.webpagetest.org/result/160428...e52d560c9/

Inspecting the requests show the H2 pseudo-header fields

Quote::method: GET
:path: /
:authority: http2.akamai.com
:scheme: https

and the User-Agent has also not been changed.

Am I missing something?
Thanks