If there is no "Show all applications" or similar button, you will need to choose "Browse" and navigate to the MS Office program folder where Excel lives.

Thank you, crankygoat, but unfortunately when I click the browse button and select Excel I get this error message.

I assume you are selecting Excel.exe. That's very odd, as that isn't the full name, nor the correct extension. Office does like to create temporary files to open, and sometimes (always?) converts some filetypes to a preferred format, but still strange. I wonder if the filename is causing problems with the spaces in it - possibly it isn't being sent with quotation marks enclosing it.

Although the csv file clearly has the Excel icon assigned to it, and Firefox likely should have defaulted to Excel, can you check in Windows Explorer that .csv and .xlsx files are set to open by default in Excel? You can right-click any file of the type and select Open With, and check or select Excel as the default program.

If that is all normal and doesn't help, with Firefox closed, you might rename the handlers.json file in your Firefox profile to handlers.json.old. Firefox will make a new one. If the old one is corrupt, this will help. You will need to choose your helper applications again. (If you have a mimetypes.rdf file, rename this also. It is the previous store FF used for the same purpose.)