Using the WebExtensions API I was unable to find anything about setting a preference in the about:config. In fact according to https://wiki.mozilla.org/WebExtensions/FAQ#Will_I_have_access_to_about:config_or_the_preferences.3F
I own the Pipelining Firefox extension, which I’m getting warnings about incompatibilities. However, it is kind of hard to update it when I can’t set about:config values. Any ideas?