[Work at Anthropic, used to work at Mozilla.]

Firefox has never required a full chain exploit in order to consider something a vulnerability. A large proportion of disclosed Firefox vulnerabilities are vulnerabilities in the sandboxed process.

If you look at Firefox's Security Severity Rating doc: https://wiki.mozilla.org/Security_Severity_Ratings/Client what you'll see is that vulnerabilities within the sandbox, and sandbox escapes, are both independently considered vulnerabilities. Chrome considers vulnerabilities in a similar manner.