Very counter-intuitive. There are better alternatives (even Firefox's own editor).

By default the state of each item is checked, which is very annoying when you are trying to work with the variables / delete variables on the spot.

You also have to (to remove it):
1. Select an item or items(but first you have to uncheck everything else or press the "uncheck all box").
2. Delete the item or items.
3. Save the configuration... what?

To make this better, make the UI a bit more intuitive for repetitive use, and maybe add some shortcuts or something.

I'll check back in the future.