Is there a reason running version 48 ? It is not even supported and does not get updates. Current ESR Version is 52.4.1 https://www.mozilla.org/en-US/firefox/organizations/all/ I suggest you install the above as it gets security updates and also uses legacy plugins. Current Firefox Version is 57 I have no idea. It is to old and not supported.

If you mean you removed the downloads button from Firefox's main toolbar, you can use Customize to restore it. See: Customize Firefox controls, buttons and toolbars.

But if you mean the actual downloads folder on your Mac, outside of Firefox, you might go to an Apple forum and see whether emptying the Trash is recoverable, or how to get the folder back from a Time Machine or other backup.

I just upgraded from a very old mac in which the downloads folder was easy to find. Not so in newer models. Turns out the folder still exists -- it just took a while to find it. An upgrade of FIrefox also resolved the loss of FIrefox access to downloads. Though, again, it took me a little while to understand the new interface. Thanks.