This should be possible. When you download FF (and I assume that you are doing that from Mozilla directly, rather than having FF update itself), move it to the desktop. Then rename it, perhaps Firefox Latest (or whatever you like). Then, move it to /Applications. It should happily coexist with the previous version in /Applications.
OS 10.14.3, iMac Retina 5K 27-inch, late 2014, 3.5 GHz Intel Core i5, 1 TB fusion drive, 16 GB RAM, Epson SureColor P600, Photoshop CC, Lightroom CC, MS Office 365