ARPU isn't a great metric here since it's revenue averaged across all users. In my experience, the vast majority of free software users sit below ARPU and are hoisted up by whales -- who are also the main reason one-off pricing like this doesn't typically exist: Mozilla would be fine if most users just paid ARPU (in fact, they'd probably make a slight profit if they could get a higher-than-industry-average free->paid conversion rate...), but they'd quickly lose their cash cows when their whales suddenly only paid ARPU instead of the 10x, 100x, 1000x, etc they already "pay".
