The following is only my opinion (:
- Webflow is not really made for you to build your designs starting with the lowest breakpoints (1)
- Webflow does such a good job in terms of responsivity that the first point doesn't matter at all (2)
- Mobile first is a marketing strategy rather than a development one (3)
(1) The way Webflow works for setting responsivity goes from the desktop breakpoint to the portrait smartphone breakpoint. Properties cascade only the way down. So you design your lowest breakpoints by setting exceptions. Also, in Webflow UI, you can't select a parent class to edit it, the designer brings you back to the desktop breakpoint when you do this.
(2) It's not a loss of time to draft your design on paper with 2 or 3 major breakpoints, including desktop, to design the desktop view first and to set your responsivity rules afterwards. Webflow is very efficient. Also by working with %/vh/vw units and by using the ROw widget in Webflow, you can reduce the work needed to make the site responsive.
(3) Mobile first means you'll concentrate on the mobile market first, so of course there's a special attention put on the lowest breakpoints, but there's not technical reason to completely ignore the others.
Hope this helps (: