It uses add-on gating similar to what they do for Web MIDI, so it's not exposed to users unless they specifically opt into it.
https://hacks.mozilla.org/2026/05/web-serial-support-in-fire...
1,452 karma · joined December 1, 2016
https://hacks.mozilla.org/2026/05/web-serial-support-in-fire...
Also, they've specifically called that out in the advisory when they're aware of that being the case. See the last out-of-band security update they released for example:
https://www.mozilla.org/en-US/security/advisories/mfsa2022-0...
https://blog.mozilla.org/blog/2017/04/10/mozilla-awards-3650...
That said, given Mozilla's current financial troubles, it seems pretty unlikely they'd be in a position to hire.
https://mozillagfx.wordpress.com/2019/10/22/dramatically-red...