Rated 4 out of 5 stars

Very nice. A great improvement idea:
the Variable List window could allow to unselect single name/value-pairs and/or alter the value. Then, in addition to the OK button a "Reload [page with altered variables]"-button could do exactly that: reloading with the new variables and data.
This would save us from the hassle of manually altering the URL in the location bar when there's need to remove or alter data.

This review is for a previous version of the add-on (0.1.4). 

You might want to check the new version with a few things implemented. I'll be pleased to read new feedback.