There is this question and others, about saving a complete webpage while online, for which htttrack works and can download a mirror of an entire site and which is one of the answers there.

But what about Save as webpage, HTML Only, when you're offline, just from the Firefox browser? To my surprise, saving even a simple webpage failed for me. This was Firefox 144.0.2 in Dec. 2025. The "Save Page As..." > "Save" button is accepted without error, but upon opening the "Display the progress of ongoing downloads" list -- it says "Failed."

I found a solution, and will share it in an answer.