When a Web Extension had the all-urls permission and made a fetch request with a mode set to 'same-origin', it was possible for the Web Extension to read local files. This vulnerability affects Firefox < 74.
74.0+build3-0ubuntu0.16.04.174.0+build3-0ubuntu0.18.04.138.8.0~repack1-0ubuntu138.8.0~repack1-0ubuntu338.8.0~repack1-0ubuntu452.3.1-0ubuntu352.3.1-7fakesync152.8.1-0ubuntu0.18.04.152.9.1-0ubuntu0.18.04.174.0+build3-0ubuntu152.9.1-1build152.9.1-1ubuntu3Exploitability
AV:NAC:LPR:NUI:NScope
S:UImpact
C:HI:NA:NCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N