To be sure, it's funky behaviour from Firefox, I would also call it a bug.
But... you must should change the MIME content-type your server is sending, and which is likely triggering the firefox bug. MJR is definitely not an executable:
C:\Temp>wget --no-check-certificate -S https://rover.jriver.com/license/my-license-code/Media%20Center30%20Windows-559151.mjr
--2022-08-29 21:49:18-- https://rover.jriver.com/license/my-license-code/Media%20Center30%20Windows-559151.mjr
Resolving rover.jriver.com... 216.14.187.183
Connecting to rover.jriver.com|216.14.187.183|:443... connected.
WARNING: cannot verify rover.jriver.com's certificate, issued by `/C=GB/ST=Greater Manchester/L=Salford/O=Sectigo Limited/CN=Sectigo RSA Domain Validation Secure Server CA':
Unable to locally verify the issuer's authority.
HTTP request sent, awaiting response...
HTTP/1.1 200 OK
Date: Mon, 29 Aug 2022 19:49:19 GMT
Server: Apache/2.4.25 (Debian)
Last-Modified: Thu, 25 Aug 2022 14:11:19 GMT
ETag: "55d-5e7115f118fc0"
Accept-Ranges: bytes
Content-Length: 1373
Keep-Alive: timeout=5, max=100
Connection: Keep-Alive
Content-Type: application/x-msdos-program
Length: 1373 (1.3K) [application/x-msdos-program]
Saving to: `Media Center30 Windows-559151.mjr'
100%[======================================================================================================================>] 1,373 --.-K/s in 0s
2022-08-29 21:49:19 (397 MB/s) - `Media Center30 Windows-559151.mjr' saved [1373/1373]
It should be application/octet-stream.