In order to change the default reader for PDF files (to not open PDF files with Firefox's internal PDF reader), follow these steps:

1. Go to Tools > Options (or Firefox > Options).
2. In the Options window, select the Applications tab.
3. In the Search field, type PDF. You should find Portable Document Format (PDF).
4. On the right handside you should find an Action column. Use that to select your favorite PDF reader. In order to view PDF files in Firefox, choose Preview in Firefox.

Did this fix your problems? Please report back soon.

Is the PDF opening in a separate (popup) window? Sites can alter which bars appear on a popup window, but the address bar should always be visible (that's the default setting). Could you check and make sure that hasn't changed?

(1) In a new tab, type or paste about:config in the address bar and press Enter. Click the button promising to be careful.

(2) In the search box above the list, type or paste window_o and pause while the list is filtered

(3) If the dom.disable_window_open_feature.location preference is bolded and "user set" to false, double-click it to switch it back to true so sites are disabled from hiding the location (address) bar on popup windows.

Thank you. I had changed the Applications>Portable Document Format Action when Firefox was overriding my Adobe Reader. Didn't even notice that it removed the URL bar. - Much appreciated!