Did you look at the file properties?

Yes. The problem is with the popup window you get when when you have the "save as" option chosen in Firefox. The default save format has been changed from .html to .htm, which doesn't work.

I've found a workaround in that I can manually add the "L" to the end of the filename extension, but another curious thing is that the first download, regardless of whether I save it as .htm or html, will ALWAYS fail. Only by then retrying the download via the downloads progress dropdown menu can I successfully save the webpage in a readable form.

I don't have this problem with Google Chrome: the default format of the "save as" popup window is html, and it will download correctly the first time. (For some unfathomable reason you can't save webpages with Edge, which a big part of why I don't use it for anything other than dealing with Microsoft.)

Another user mentioned having to use Save Page As twice when using the "Web page, complete" (HTML file + folder) format:

When I Use the Save Page As menu option, it no longer saves the page properly

I don't know whether the file extension is a factor for him as well.

You can inspect the MIME database key with the registry editor (regedit.exe) and do a search for that MIME type (file extension) via Ctrl+F. Be cautious with editing the registry as there is NO UNDO possible: all changes are applied immediately. You can export key(s) in the registry editor before making changes. You can check specific file extension keys (e.g. .html) in the registry with the registry editor.

• HKEY_CLASSES_ROOT\.xxx

You can check a possibly linked MIME type in the MIME Database registry key.

• HKEY_CLASSES_ROOT\MIME\Database\Content Type\

I think I'll just manually enter the "l" at the end of the extension and retry the download every time rather than muck about with the registry. Or use Google Chrome, because it still works fine with that browser, so this is clearly a Firefox glitch.