Around 80% of Firefox users are on Windows, per https://data.firefox.com/dashboard/hardware

That same site also suggests that Firefox has around 200e6 monthly active users, the average user uses Firefox 3.5 days a week, and for 5.5 hours per day.

My math could be wrong, but taking the above into account, and arnaudsm's 5 W estimate, I come up with an upper bound of around 80 MW. Discount that further by whatever proportion of Windows users you assume were actually affected. Not a whole coal power plant, but nothing to sneeze at.