7.5M out of 193M is 0.39%, so it's off by about a half. Hardly a "tiny fraction". Presumably the growth in internet users in the period would increase the relative decline in market share, too.
You're comparing apples to oranges. The 0.75% decline from the OP is of market share across all browsers. The figure here is a 3.9% (not 0.39%) decline in Firefox users only. This would translate to 3.9% of 3.79% = about 0.14% of market share. It's not about half but more than 5x less.
