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Firefox has a new version of the containers.json that needs to be read to extract the correct cookies. E.g. the follow command currently fails even though the default container user-context-personal exists:

yt-dlp --cookies-from-browser firefox:aaaa9aaa.default::user-context-personal "https://www.youtube.com/playlist?list=UUMOLe_q9axMaeTbjN0hy1Z9xA"
# Extracting cookies from firefox
# ERROR: could not find firefox container "user-context-personal" in containers.json

This is because version 5 of the contains.json format has the member l10nId (note the small d) with the format user-context-${name}, where version 4 had l10nID with userContext${name}.label. Compare the version 4 from an old issue comment on this repo with the following version 5:

{
  "version": 5,
  "lastUserContextId": 31566,
  "identities": [
    {
      "userContextId": 1,
      "public": true,
      "icon": "fingerprint",
      "color": "blue",
      "telemetryId": 1,
      "l10nId": "user-context-personal"
    },
    ⋮
  ]
}

This PR fixes this by still supporting version 4, but in addition supporting version 5 by testing one extra regular expression.

I was unable to find any documentation of the different versions of containers.json, I assume Firefox sees this as an internal detail only and not part of any public API.