No, Firefox vendors everything it uses. Debian has no patches wrt vendoring of Rust code in Firefox. Source: I work for Mozilla and maintain the Debian Firefox package.
Thank you, I appreciate the correction. Did this change at some point in the past? I seem to remember the Arch Linux package downloading a ton of Rust stuff at one point.
No, it has always used vendored crates. Well, technically, it wasn't using vendored crates until https://bugzilla.mozilla.org/show_bug.cgi?id=1298422 , but before that, there were only a limited number of crates in use, and their only dependencies were local, so practically, that's the same thing.