It works well, but it's not well explained: it will not work at all if you do not first go in the ShareX "Application Settings">Integrations and activate the "Enable Firefox Addon Support".
I thought the app was useless until I combed through the settings to find that option: I would advise to write this crucial step in the description.

I expected more functionality. Good stuff anyway.

Quite useful if you want to mirror an image.
It would be nice to have the possibility to edit the picture uploading it though.