Hi, please check your Downloads setting - Startup, home page, tabs, and download settings

Also see: Manage file types and download actions in Firefox

If your question is resolved by this or another answer, please take a minute to let us know. Thank you!

Thanks. I had already looked on those pages and doesnt resolve the issue. I have since discovered it is just PDF files that have been affected as Excel hasnt changed. I downloaded Adobe update yesterday so must have been something in that update

I had the same problem with Excel. My settings have always been "Always ask", but somehow the setting had changed to "Save" sometime very recently.

Hi brynmorgan, check your settings on the Options page, Applications section. This article will get you to the spot (even if the title isn't exactly what you're aiming for): View PDF files in Firefox or choose another viewer.

Hi MichaelArmstrong, I don't know why the Excel file handling would change. The reason PDF settings may change is that the Adobe Acrobat plugin is no longer enabled in the regular release of Firefox 52 and later. (No plugins other than Flash run in those versions.) Users who were automatically directing PDFs to that plugin would have an invalid setting that might not get switched back to the built-in PDF Viewer for one reason or another. But Excel files never opened in Firefox, so... ???

Sometimes the settings file which stores these application handling preferences becomes corrupted. Since it has a somewhat inscrutable XML-based format, we generally recommend letting Firefox rebuild it from scratch. And this is how...

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

• "3-bar" menu button > "?" button > Troubleshooting Information
• (menu bar) Help > Troubleshooting Information
• type or paste about:support in the address bar and press Enter/Return

In the first table on the page, click the "Open Folder" / "Show in Finder" / "Open Directory" button. This should launch a new window listing various files and folders.

Leaving that window open, switch back to Firefox and Exit, either:

• "3-bar" menu button > "power" button
• (menu bar) File > Exit / Quit

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

Start Firefox back up again. PDFs should be assigned to "Preview in Firefox" by default, and Excel files should be "Always ask" by default.

I hate it when "may become corrupted" is given as an excuse/reason for a problem. But the rebuild instructions were useful; thanks.

MichaelArmstrong said

I hate it when "may become corrupted" is given as an excuse/reason for a problem. But the rebuild instructions were useful; thanks.

Probably if all explanations were known it would have been prevented by now. If you search around for "mimetypes.rdf" you can find a lot of strange behaviors in threads over the years.