That's strange. The built-in viewer -- default viewer -- is described with the phrase "Preview in Firefox" and you would have to have changed it to "Use Adobe Acrobat (in Firefox)".

Well... you or an add-on or another program on your system would have to have changed it.

What if you disable the Adobe Acrobat plugin (not the Create PDF extension, leave that enabled). Here's how you can try that:

Open the Add-ons page using either:

• Ctrl+Shift+a
• "3-bar" menu button (or Tools menu) > Add-ons

In the left column, click Plugins. On the right side, find "Adobe Acrobat" and change the permission to "Never Activate".

Then on the Options page, Applications panel, change your preference for "Portable Document Format (PDF)" to "Always Ask" so you get the download dialog instead of the PDF opening in a tab.

Does it stick?

Disabling Adobe Acrobat worked - thank you sir!