Create README.MD

Firefox/README.MD

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+# Firefox - prefs.js
+
+# Windows [click to download firefox esr](https://download.mozilla.org/?product=firefox-esr-latest&os=win&lang=zh-CN)
+```
+%AppData%\Mozilla\Firefox\Profiles
+```
+
+# MacOS [click to download firefox esr](https://download.mozilla.org/?product=firefox-esr-latest&os=osx&lang=zh-CN)
+```
+~/Library/Application Support/Firefox/Profiles
+```