How to know font of the current web page in Firefox?
1. View source (right click > View Page Source or Ctrl+U) and find the font from the style sheet.
2. Install Firebug, find the DOM element, and look at its style.
Neither answer applies to current versions of Firefox, so I am posting this answer.
FireBug is not available as a Web Extension, and Firefox devtools have been updated.
Current Firefox versions allow you to right-click on any element and select Inspect Element. Then, click the Inspector button on the devtools toolbar, and select the Computed tab on the top right of the devtools. Look or search for