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Alpha o - I respectfully suggest that you use a mainstream browser. They can all save HTML, and there are even add ins that will create an EPUB directly... Grabmybooks, dotEPUB come to mind.
The problem with 'optimised' bric-a-brac like Pale Moon is that a feature that was removed to create the so-called optimal browser will often as not be the very feature one needs to get a job done.
This shows you the best formats to convert ==>>
What are the best source formats to convert? - PDF is at the bottom of the list.
BR