All versions of Firefox include built-in support for PDF handling that doesn't rely on external add-ons like Adobe Reader. The built-in PDF handler is currently unsupported for QuickBooks Online, but you can opt to use it at your own risk. We are testing this functionality to determine if we can officially support it for printing PDF forms in QuickBooks Online.
If you'd like to go back to your usual PDF handler:
Go to the Firefox menu, go to Tools or Options, then click Options (OS X: Go to the Firefox menu and click Preferences).
Click the Applications tab.
Change Portable Document Format (PDF) to Adobe Reader or your PDF handler of choice.