tl;dr - would you please consider/look at including Firefox's
BuildID, output as a string, in Marionette's
BuildID variable in application.ini https://searchfox.org/mozilla-central/rev/e00ea598e52bbb35f8c45abf9c2eade17962bb5e/build/application.ini.in#29
WebPageTest, via its wptagent, gets it from Marionette's
capabilities object as the string
browserVersion, with a sample value "67.0a1".
Since Firefox 64,
navigator.buildID() no longer works, according to https://developer.mozilla.org/en-US/docs/Web/API/Navigator/buildID, so, even if it were to get it via documented means I've come across, we'd get something like this fixed one:
Perhaps it could be added as a vendor-prefixed string -- moz along with the others, e.g. https://searchfox.org/mozilla-central/rev/e00ea598e52bbb35f8c45abf9c2eade17962bb5e/testing/marionette/capabilities.js#409-416?
For more context, I'm close to submitting my WebPageTest perf-runs to the Mozilla Telemetry Data Pipeline (https://github.com/mozilla/wpt-api/pull/236), and an accurate browserVersion/browser_version is key (no Python-dict pun intended, but I'll take it).
Here's what we have (will have): https://github.com/mozilla/wpt-api/pull/236/files#diff-6d5e2d2375d56dbb4c662b976106321dR65
(Also helpful would be the SHA commit hash for mozilla-central for each build, available via about:buildconfig, and has a value like:
https://hg.mozilla.org/mozilla-central/rev/de51545099a617602be187d1c0f68ff2a87d6fb2 but that's totally another bug, another day.)