Bug 1583388 - Recommend the Firefox debugger VSCode plugin. r=jlast

DONTBUILD because this is just an IDE config change.

Differential Revision: https://phabricator.services.mozilla.com/D46874

--HG--
extra : moz-landing-system : lando

.vscode/extensions.json

@@ -14,6 +14,8 @@
     "rust-lang.rust",
     // CSS support for HTML documents.
     "ecmel.vscode-html-css",
+    // Web app and extension debugging.
+    "firefox-devtools.vscode-firefox-debug",
     // IDL language support
     "mythmon.idl"
   ]