chore(auth): Add firefox client id to OAUTH_TOKEN_EXCHANGE_CLIENT_IDS

[dschom]

Because

• Firefox team needs this to test token exchanges.

This pull request

• Adds firefox client id to the default list.

Issue that this pull request solves

Closes: FXA-14122

Checklist

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• My commit is GPG signed.
• If applicable, I have modified or added tests which pass locally.
• I have added necessary documentation (if appropriate).
• I have verified that my changes render correctly in RTL (if appropriate).
• I have manually reviewed all AI generated code.

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