Firefox started using PGO on Windows in Firefox in 2008 [1], Android last year [2], and macOS earlier this year [3]. Firefox uses PGO on Linux, but I don't remember when it was enabled.
[1] https://bugzilla.mozilla.org/show_bug.cgi?id=418865
[1] https://bugzilla.mozilla.org/show_bug.cgi?id=418865
The all-new Firefox for Android (codename Daylight) is lightening fast, and perceptibly seems on-par with the Chrome for Android in terms of responsiveness.
What stumps me is Chrome for Android is 3.8MB whereas Firefox Daylight is upwards of 40MB.