A service worker can send the activate event on itself periodically which allows it to run perpetually, allowing it to monitor activity by users. Affects all versions prior to Firefox 60.
Exploitability
AV:NAC:LPR:NUI:NScope
S:UImpact
C:HI:NA:N7.5/CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N