Case #1: FF v59.0.1
   No failure is signalled, the page saves OK, and can later be loaded from local disk as expected when offline.
 Case #2: FF v90.0.2
   The download icon in the toolbar has a yellow triangle, and when clicked it displays a status panel saying that the download "Failed", and offering to "Retry download".
     Case #2a: If I do NOT click the "Retry download" icon, and just terminate FF, disconnect from the internet, then restart FF and load the page offline, all is OK as per Case #1 above.
     Case #2b: If I DO click the "Retry download" icon, then FF retries, says "Completed", and clears the yellow triangle from the icon.  BUT the saved page does not display correctly when offline, because in the saved .htm all the references to image files, .css files, .js files, etc. are to the original web URLs, not to the local <Page>_files/*.* files (which do exist).

As already reported by numerous people, the newer versions of FF sometimes don't save some web pages properly to local disk for later offline use. Specifically, the saved .htm file references the original URLs instead of the Page_files/*.* local copies. FF v59.0.1 did not have this problem, which started when I upgraded from v59.0.1 to v80. v90.0.2 still has this problem. The problem occurs with many web pages, but not all. e.g. I don't have this problem saving pages from support.mozilla.org, but I do with many News sites, and others such as the example below: Example (this was all done within 3 minutes, and is repeatable): Web page: [https://www.daz3d.com/dforce-summer-chic-suit-for-genesis-8-females A page in the store at DAZ3D] File / Save as '''Case #1''': FF v59.0.1 No failure is signalled, the page saves OK, and can later be loaded from local disk as expected when offline. '''Case #2''': FF v90.0.2 The download icon in the toolbar has a yellow triangle, and when clicked it displays a status panel saying that the download "Failed", and offering to "Retry download". '''Case #2a''': If I do '''NOT''' click the "Retry download" icon, and just terminate FF, disconnect from the internet, then restart FF and load the page offline, all is OK as per Case #1 above. '''Case #2b''': If I '''DO''' click the "Retry download" icon, then FF retries, says "Completed", and clears the yellow triangle from the icon. '''BUT''' the saved page does not display correctly when offline, '''because in the saved .htm all the references to image files, .css files, .js files, etc. are to the original web URLs, not to the local <Page>_files/*.* files''' (which do exist). An example of the corresponding lines in the 3 saved .htm files: Case #1: <link rel="stylesheet" href="dForce%20Summer%20Chic%20Suit%20for%20Genesis%208%20Female(s)%20%20%20Daz%203D_files/design-b24fd175375034500aabab482fb3a1a3.css" about="0"> Case #2a: <link rel="stylesheet" ref="dForce%20Summer%20Chic%20Suit%20for%20Genesis%208%20Female(s)%20Daz%203D_files/design-b24fd175375034500aabab482fb3a1a3.css" about="0"> Case #2b: <link rel="stylesheet" href="/groups/design-b24fd175375034500aabab482fb3a1a3.css" about="0"> In Case #2b, the string "_files/" occurs nowhere in the .htm file. Another example of differences between the saved .htm files - these are the last lines before the </body> token: Case #2a: <script async="" src="dForce%20Summer%20Chic%20Suit%20for%20Genesis%208%20Female(s)%20Daz%203D_files/persist.js"></script><script async="" src="dForce%20Summer%20Chic%20Suit%20for%20Genesis%208%20Female(s)%20Daz%203D_files/check.js"></script> Case #2b: <script defer src="https://static.cloudflareinsights.com/beacon.min.js" data-cf-beacon='{"rayId":"67a32d66eefdfb88","token":"90626adc84c24536bed8adeb58ff8fdb","version":"2021.7.0","si":10}'></script> The Case#2a .htm file does not refer to "beacon.min.js" at all. The Case #2B .htm files does not refer to "persist.js" or "check.js" at all. (Maybe the problem occurs only on CloudFlare-based sites?) I hope Firefox can fix this soon. TIA.