That is good question. In very initial project phases, I try to work on both simultaneously before making crucial decisions — e.g. how tables and grids will work, if they will add horizontal scrollbar or degrade the number of columns etc.

From that moment it depends on the priorities. From all the enterprise software that I was working on, the mobile was not really a productivity booster solving real user goals but rather a window into the future. Thus I am focused on the desktop-first and make sure it also works on mobile.