How to Save Website Pages for Offline access in Firefox/...?
You can try HTTrack Website Copier. Seems to match your needs fully, not tried by me :

HTTrack is a free software (GPL) offline browser utility, allowing you to download (copy) a website from the Internet to a local directory, building recursively all directories, getting html, images, and other files from the server to your device.
HTTrack arranges the original site's relative link-structure. Simply open a page of the &quot;mirrored&quot; website in your browser, and you can browse the site from link to link, as if you were viewing it online.

Edit Tested it now

All sites do not permit copying - this site doesn't:). For reasons see FAQ

Can be very time consuming when on WiFi. I tried on Amazon India and aborted after 7 minutes . It copied the first page fully, it looks like, but you can set up rules

I tried on a smaller banking website and it was able to copy fully with three levels of linking


You have to test and see if it works for your sites
Refer comment. Copied the content of LG TV as shown below. Note the download link location in internal storage and the third one shows internal link opened up. Took one minute