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.

這個真心好用,簡約而不簡單.

As close to perfection as God allows, just like the desktop application this enhances. I'm very grateful to the developer and recommend this to anyone.

Unfortunately, it doesn't work for me at all. Nothing happens after clicking on the entry in the context menu for text, pictures and videos.