Is the problem that Firefox opens a blank tab, or that it forces you to download the file?

I assume you've tried switching viewers between the internal viewer and the Adobe plugin. If not, this article has the steps: View PDF files in Firefox or choose another viewer. Sometimes switching back and forth will correct an error in the settings file.

You also can rename or delete the settings file that stores these associations and have Firefox regenerate it. If the file is corrupted, this might be the only option that works. Here's how:

Open your current Firefox settings (AKA Firefox profile) folder using

Help > Troubleshooting Information > "Show Folder" button

Leaving that window open, make any Firefox window active and Exit the program (e.g., File menu > Exit or Firefox menu > Exit)

Pause while Firefox finishes its cleanup, then rename mimeTypes.rdf to something like mimeTypes.old

Restart Firefox. Your PDF preference likely will be reset to the internal PDF viewer, but you can switch it to the Adobe plugin as described in the above-linked article.

Any improvement?

I thought I got rid of the mime file the first time. However, I never exited out of Firefox first. When I properly exited out of Firefox and then deleted the mime file, it fixed the problem of downloading instead of displaying. Thanks for your help!

Thanks for posting back that you fixed the problem. I marked jscher2000's reply as the solution.

I already fixed it. I never exited out of Firefox before deleting the mime file. it works now.