When it comes to the HTML mode, you should note that this is not the same editable area as the Design mode.
Using the Find And Replace tool in HTML mode is available and supported by the browser only in Chrome. Also, it appears that due to browser limitation the Replace All functionality does not work in HTML mode.
The tool is designed to work properly in Design mode. Due to browser limitation there is no avilable approach for this tool to work in textarea (HTML mode) and in editable iframe (Design mode) at the same time. Also I cannot provide a possible solution to handle this matter.
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