概要

WEBGL_debug_renderer_info is deprecated in Firefox and will be removed. Please use RENDERER. Build.loader.js:1:9894
[UnityCache] Failed to load 'http://localhost:51173/Build/Build.data.br' from indexedDB cache due to the error: TypeError: NetworkError when attempting to fetch resource. Build.loader.js:1:14524
Failed to download file Build/Build.data.br Build.loader.js:1:3110
Applying workaround to Firefox bug https://bugzilla.mozilla.org/show_bug.cgi?id=1397977 Build.loader.js:1:6569
navigator.mediaDevices support available Build.framework.js.br:3:4755
Uncaught (in promise) TypeError: e is undefined
    l http://localhost:51173/Build/Build.loader.js:1
    promise callback*createUnityInstance/l/< http://localhost:51173/Build/Build.loader.js:1
    callRuntimeCallbacks http://localhost:51173/Build/Build.framework.js.br:3
    preRun http://localhost:51173/Build/Build.framework.js.br:3
    run http://localhost:51173/Build/Build.framework.js.br:3
    runCaller http://localhost:51173/Build/Build.framework.js.br:3
    removeRunDependency http://localhost:51173/Build/Build.framework.js.br:3
    receiveInstance http://localhost:51173/Build/Build.framework.js.br:3
    receiveInstantiationResult http://localhost:51173/Build/Build.framework.js.br:3
    promise callback*unityFramework/createWasm/instantiateAsync/< http://localhost:51173/Build/Build.framework.js.br:3
    promise callback*instantiateAsync http://localhost:51173/Build/Build.framework.js.br:3
    createWasm http://localhost:51173/Build/Build.framework.js.br:3
    unityFramework http://localhost:51173/Build/Build.framework.js.br:3
    l http://localhost:51173/Build/Build.loader.js:1
    promise callback*l http://localhost:51173/Build/Build.loader.js:1
    createUnityInstance http://localhost:51173/Build/Build.loader.js:1
    createUnityInstance http://localhost:51173/Build/Build.loader.js:1
    onload http://localhost:51173/:107
    EventHandlerNonNull* http://localhost:51173/:106
Build.loader.js:1:5497

Unity で WebGL ビルドして Firefox で動作確認しようとしたら
上記のエラーが発生する現象に遭遇した

Project Settings の「Player > WebGL > Publishing Settings」を開いて
「Decompression Fallback」をオンにするか

「Compression Format」を「Gzip」にしてからビルドしたら
エラーが発生しなくなった