Modify moat client code so it is compatible with ESR68

While working on #29430 (moved), Kathy and I found some incompatibilities in Tor Launcher's moat client code (src/modules/tl-bridgedb.jsm). All of the problems are due to Firefox internal API changes. While working on #31300 (moved), we did not exercise this code so we did not find these problems then.

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