Sorry, I don't know how to name that, tried to search looking for rulers, guides, grid and markup - no luck.
I'm speaking about blue dashed lines and the gray area. Usually when you're at inspector you move mouse over the code, and FF highlights the proper area. Now, after the area was selected, I was playing with colour (right pane with styles) to change it. Then, that grid remained persistent whatever I did and switched off only when I switched off the developer console, but this is not the way I'd like to have.
Update: Somehow it is related with style editor, because it highlights in the same way all areas with that style, not this particular only.
Update: FF 49.0.1, it goes off when close the developer tool or just reload the tab.
So how it can be switched off?