You click on a .pdf file; Firefox won't load and says "not a pdf or corrupted," but all other browsers open it normally

Clicking on a link to a .pdf file in Firefox results in Firefox displaying the box "Format error: not a .pdf or corrupted"

This has happened for several years with any number of web pages. A link to the latest event is below, but it typically happens almost every time I click on a link to a .pdf