Security issues of Firefox:
lack of site isolation (Project Fission - MozillaWiki)
CFI, (510629 - (cfi) [meta] Ship Control Flow Integrity (CFI))
ACG (https://bugzilla.mozilla.org/show_bug.cgi?id=1381050)
CIG (1378417 - [meta] Support Binary Signature Policy and eventually Code Integrity Guard on Windows)
win32k lockdown (https://bugzilla.mozilla.org/buglist.cgi?quicksearch=win32k)
lack of Linux/macOS gpu isolation (Security/Sandbox/Process model - MozillaWiki) ???
lack of a hardened malloc (PartitionAlloc Design)
complete lack of any sandboxing whatsoever on Android (https://bugzilla.mozilla.org/show_bug.cgi?id=1565196)
Most of these issues are decade old which is really concerning.