That doesn't work on my FF if I hover my mouse over the top left corner of any element row column or otherwise, all I get is what it is I am hovering over, which is ok if thats the element I want to work on, but say I want to alter the width of that row, I have to go into the Inspector (regardless of the %) and scroll up and down in that until the particular row is highlighted, then double click it to get into design view.
Another for instance I have a row/column in that column are a number of containers, one container holds a H3 element and a paragraph, If I want to do something with that container, I cannot get to it until I go to the Inspector, and scroll through until it highlights. (even selecting the paragraph and then going to the inspector doesn't open with the paragraph highlighted, it opens at the top. same goes for the row that holds all I can't get to it unless I go to the inspector and scroll up or down for it to highlight.
It's not a great issue, just would be nice to be able to choose, it looks like maybe there is something wrong with my install, because from your screenshot thats exactly what I am trying to describe (poorly) but that just doesn't work on my FF