The latest version of Firefox opens PDFs natively. These are the instructions on how to update Firefox.

Update Firefox to the latest version

By default, Firefox is set to automatically update itself but you can always do a manual update. Here's how:

1. On the menu bar click the Firefox menu and select About Firefox.
2. The About Firefox window will open and Firefox will begin checking for updates and downloading them automatically.

3. When the updates are ready to be installed, click Restart to Update.

If the update didn't work for some reason, just download and install a fresh copy.