It is, unfortunately, only really an option that’s particularly feasible for technically-inclined people. Which is a shame, because it speeds up the web so much, and makes a great many pages work much better—but it breaks a fair bit too. (Those are the main reasons I started doing it a few years ago, rather than privacy and such, which are secondary reasons at most to me.)
The sad story: it’s the technically-inclined people that have powerful machines despite being capable of making do with weak machines, knowing better how to manage limited computational resources, but it’s the technically-ignorant people that have weak machines and need more powerful machines.
In other news, after thinking of it from time to time for a few years, yesterday I finally disabled all font selection to see what it’s like, so now pages can only choose monospace, sans-serif or serif (I choose my own fonts for those years ago—all this font stuff is in Firefox’s preferences, BTW, no extension required). It’s rather pleasant, so far, a great improvement, with only very minor trouble from icon fonts.