openmw.org

I want to import my main save from Morrowind. openmw-essimporter.exe doesn't boot, it shows a command prompt for a couple frames then closes.

You need to run openmw-essimporter from command prompt. First you open command prompt then you go to the directory containing openmw-essimporter.exe. A quick search I found the below. Also, the importer is not complete. Have a look at the latest page below detailing the current issues.

Save importer not opening? - openmw.org
.ess Importer - Page 5 - openmw.org
Help with using OpenMW save importer in Command Prompt? : Morrowind

Note that the ESS Importer is incomplete. The OpenMW save file it produces will have game-breaking bugs that prevent you from doing much.