Google Chrome and other Chromium-based browsers had a random password generator feature since last year's fall, so the fact that Firefox is getting it as well should not surprise anyone. This feature is already available in the latest Nightly version of Mozilla's browser and should hit the Stable branch this fall.
https://www.notebookcheck.net/Firefox-should-get-a-random-password-generator-this-fall.426505.0.html