Unfortunately Firefox has a bug that does not allow to upload exported color schemes.
If you don't like to use built-in sync then you can try to do it manually:
1) Go to about:debugging#/runtime/this-firefox
2) Find Midnight Lizard and press Inspect
2) Paste into the Console "chrome.storage.sync.get(null,x=>console.log(JSON.stringify(x)))" without quotes and press Enter
3) Copy results from the Console to the file.
4) Open about:debugging#/runtime/this-firefox on your another device
4) Paste into the Console "chrome.storage.sync.set(JSON.parse('replace it with the text from the file and keep single quotes'))" without quotes and press Enter

PS: if you have disabled "Sync settings across your devices" in Midnight Lizard settings then you need to use "chrome.storage.local" instead of "chrome.storage.sync"