View Source lets you look at the HTML or XML source for the page you're viewing. To activate View Source:

   context-click in the page and select View Page Source
   press Ctrl + U on Windows and Linux, or Cmd + U on macOS

The command opens a new tab with the source for the current page.

What is html or text view

I what context? OR Where are you seeing that?

For instance, with Gmail that would be about how an email is to be displayed. Simpler view for older, slower PC's and for users who prefer "less bells & whistles" such as animated images.

Note that there is also Reader View.

• https://support.mozilla.org/en-US/kb/firefox-reader-view-clutter-free-web-pages