That is still more than an order of magnitude higher (2.17% rather than 0.09%).

And FF ESR is the more common by far (44.5%) as it is the default, people only install the other FF package if they have specific need to be closer to the leading edge. The build environments for the two will be very close, at least compared to the oddities being reported in the Guix requirements, so the hassle of maintaining FF and FF-ESR compared to just one of them is not going to be comparatively large compared to maintaining packages for just one of the pair.