The Firefox web browser does not have the capability to save the contents of a webpage to a single file, such as an MHTML file, which is a web archive file format that allows one to save the HTML code, images etc. on a webpage into a single file. Though the capability is absent from Firefox, you can add the functionality of saving a page to a single file by installing Save Page WE, an add-on for Firefox by DW-dev. The extension provides the capability to "Save a complete web page (as currently displayed) as a single HTML file that can be opened in any browser. Save a single page, multiple selected pages or a list of page URLs. Automate saving from command line." The saved file is not an MHTML file, or one of the other common web archiving file formats, but it is a single HTML file that Firefox and other browsers can read. Save Page WE is implemented using the WebExtensions API and is available for both Firefox and Chrome with identical functions and user interfaces.
When you start the installation process, you will see a list of the permissions required by the extension:
Click on the Add button to add the extension. You should then see the message "Save Page WE was added. Update permissions and data preferences any time in the extension settings." You also have the option to pin the extension to the toolbar.
Select the option to pin the extension to the toolbar, which will result in an icon of a 3.5" floppy disk appearing on the toolbar. You can click on that icon to save the page displayed in the current browser tab.
You can also save a page by hitting the Alt+A key combination (you can cancel the saving of a page with At+C). Though the file format is MHTML, the file extension used by SavePageWE is .html, which Firefox can open; it is not an MHTML file and if you change the extension to .mhtml, Firefox will prompt you to use another application to open the file.
If you check the options for the extension, you will see there are a lot of options to the extension that you can modify.
Anyone who wishes to have the ability to save a webpage as a single file added to Firefox, can add their voice to the post at Add native Web Archive File support to Firefox, including MHTML, MAFF, and Webarchive...
References