I was also able to reproduce the issue in a local `--enable-debug --enable-optimize=-O1` build (with a fresh profile, using the same error page viewing localhost:8000 with no web server listening there, while compiling another Firefox build in the background).
Bug 1684415 Comment 19 Edit History
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I was also able to reproduce the issue in a local `--enable-debug --enable-optimize=-O1` build (with a fresh profile, using the same error page viewing localhost:8000 with no web server listening there, while compiling another Firefox build in the background). No errors showed in the error console.