If you used to save sometimes a web page with Internet Explorer as a single archive with text and graphics (mht) and can’t open them with Firefox, you need an add-on such as Mozilla Archive Format.
Mht files are usually created by Internet Explorer. The Mozilla Archive Format add-on can read and create nht files, with excellent compatibility.
Note that this add-on, in recent Firefox versions, faces compatibility problems and may disable the “Save Image As” right click option.
UnMHT is another add-on that will let you view Mht (Mhtml) files, and save complete web pages, including text and graphics, into a single Mht file in Firefox/Mozilla Suite.
You cannot save to Mht from the built-in save dialog by default. To have this feature, you need to install the Mozilla Archive Format. You can save a web page in Mht format using UnMHT by right clicking on that page and selecting the option “Save as MHT”.