html.elements.link.rel.preload.as-track: add Firefox bug URL (#29362)

Issue: https://bugzilla.mozilla.org/show_bug.cgi?id=1886145

Author: ddbeck

html/elements/link.json

@@ -1459,7 +1459,8 @@
                   "chrome_android": "mirror",
                   "edge": "mirror",
                   "firefox": {
-                    "version_added": false
+                    "version_added": false,
+                    "impl_url": "https://bugzil.la/1886145"
                   },
                   "firefox_android": "mirror",
                   "oculus": "mirror",