It’s no longer possible for extensions to modify that window in Firefox, but you can use the Zotero button when viewing the PDF in Firefox to download the PDF directly to Zotero.

When I try to open the pdf (after I choose Zotero as program to open) I get an error message that says the file format is not supported.

Yes, this is what I was trying to do from the FAQ page: "find the article page instead and use the “Save to Zotero” button to download the metadata and PDF automatically (recommended whenever possible);" What is the "article page"? I thought this was what I was trying to do.

Ok, I think I see what the problem is. The zotero button in my toolbar is not working. it is not detecting the type of data (the icon doesn't change, it shows a blank page) and nothing saves to my account when I hit the button. I read the FAQ and have checked the add-on. The Zotero connector is listed in my add-ons.

In Zotero 5, there is only one option in the Zotero button in the toolbar of your navigator. Instead, you should use the right button of your mouse and follow these consecutive options: Zotero Connector -> Save to Zotero -> (and then select the desired option)

Start here: https://www.zotero.org/support/troubleshooting_translator_issues

You don't need to right-click on anything here.