How do developers make money from freeware?

I’m thinking along the lines of free mmo games but this question can apply to any type of freeware.
So how do the freeware devs make lots of money, or even just enough money to sustain there product thats needs constant attention i.e firefox web browser, anti viruses software like avira and avast, mmorpgs with severs like maplestory?

I know some software companies give out freebies like Microsoft and Adobe but they’re already rich, I’m talking about regular indi developers.

Thanks.

They often ask for donations. They often use words like "If you find this useful, Please donate $1" or some other small amount. It helps when they make it easy and safe to pay like with paypal. If the program goes viral, It can make a good return. There are also link tie in services like "clickbank" that generate income by you drawing people to your website, then the link distracting them away to other websites that pay you a commission if a sale is generated. The amounts are small per click, but under a lot of high traffic, it is possible to make money.