Ed Cryer <
ed@somewhere.in.the.uk> wrote:
I have a stray .MHT file, set to open with MS Edge. When I try to open
it with Firefox it won't let me; it continually asks what to do with it.
It is RFC-822 data (991 KB).
It also opens in MS Word.
MHT is a Microsoft-proprietary file format. Well, the structure of the
file is defined by Microsoft, but content uses headers and MIME blocks.
See:
https://docs.fileformat.com/web/mht/
https://en.wikipedia.org/wiki/MHTML
Requires a handler to render it. MHT is an archive file; however,
again, it is proprietary, so 7-Zip, Peazip, and other archive tools
can't open it.
There are MHT viewers you can use to view the contents; for example:
https://apps.microsoft.com/store/detail/mht-viewer/9NBLGGH2VSQ9
Despite you don't have Internet Explorer which had the option to save a
web page as a .mht archive (instead of a folder for the main web page,
and subfolders for all resources aka Web Complete save), you should
still be able to open the .mht file in MS Edge provided you're not the
type to purge Edge from your Windows 10 setup.
Firefox does not have MHT support. Never did. It's not a standard
archive file format. Again, Microsoft proprietary, and why Edge still
supports MHT. No surprise MS Word opens a .mht file since it's
Microsoft supporting a Microsoft-proprietary format.
Add-ons are where extra support is found, like reading .mht files. For Firefox, try the UnMHT add-on.
https://addons.thunderbird.net/en-us/firefox/addon/unmht/
Alas, dev got abandoned on that add-on a while ago. If you click on the
link to show older versions, those run on pretty old versions of
Firefox, like back to FF v55 which probably means the add-on uses the
XUL/XPCOM model that got dropped from FF after version 57 when Mozilla
went to the then-new WebExtensions (WE) add-ons programming model.
You didn't mention which version of Firefox you are using. There are
some forks of Firefox that still support the old XUL/XPCOM extension programming model. I think Waterfox is one, but I have not kept up with
which FF deviants, er, variants that are still currently supported, and
which you might still have to get old versions to use old extensions.
Use Edge to view .mht files. Microsoft still supports their proprietary archive file format in Edge. I think Chrome and Opera also (still) open
MHT files. Forget viewing MHT files in Firefox.
There are standalong MHT viewers, like:
https://www.freeviewer.org/mht/
https://www.bitrecover.com/mht/viewer/ (free & pay versions)
https://www.perfectdatasoftware.com/mht/viewer/
There are also online MHT viewers, but I didn't bother to find and
mention those.
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