Is the SD Card slot completely nonfunctional, for example, when you insert a card, Windows behaves as though nothing happened? Or does Windows fail to read the contents and insist that the card is blank?

If the problem is limited to Firefox, how are you trying to access the card -- uploading to a website?

Card is not read at all after I have fully inserted it. It used to pull up how to read the the card / what to do with it. Then you could open the card and get a file listing. Now it is like I haven't done anything.

Also, this happens whenever I try to read a card - and I have tried multiple cards. I just have noticed it after upgrading Firefox.

Hmm, I'm not sure we can help you here. You could try Dell's forums, or a more general forum that addresses hardware issues.

Doesn't the reader show up in Windows/File Explorer?

Is this a built-in reader or one attached via an USB interface?

Did you check the BIOS set up?

No, the reader is no longer showing up on Windows File Explorer and it used to. It is installed with / came with the laptop; it is not an add-on. It has been reading fine until now and the only intentional change I have done is allowed Firefox to update. Is there a way to back out that Firefox update (without removing the whole program) and see if that resolves the issue? There should be no issue with the BIOS as I haven't changed it.

Firefox primarily uses two folders:

C:\Program Files (x86)\Mozilla Firefox C:\Users\username\AppData\Roaming\Mozilla\Firefox (back up this folder)

Also for updating purposes, this service:

C:\Program Files (x86)\Mozilla Maintenance Service

And cached and most other temporary files are here:

C:\Users\username\AppData\Local\Mozilla\Firefox

It's hard to see why changes in these folders would affect your SD Card reader, but if you decide to remove Firefox as a test, I have noted the one that you need to back up somewhere safe. See also: Back up and restore information in Firefox profiles.