XULRunner error on W 8.1 but reinstall fails to solve

Using FF 75.0 and SB 5.31.6 64b runs fine on my W10 pc but today on another W 8.1 laptop it gave the error XULRunner, Error: Platform version 75.0 not compatible with minVersion  74.0  maxVersion  74.0. I reinstalled FF and FB. Then gave the same error. But this afternoon the new shortcut opens SB + FF fine.

Just wondering why it took a number of hours to run correctly after I deleted old settings (no add-ons) and reinstalled both appl's. Odd.

 

Thanks,

 

Mac