Mozilla needs to "rent" the browser, just like every other application or app on the market today. $5/mo, or $50/yr. They could continue to give away a version that, say, doesn't allow plugins or something.
* Hopefully this doesn't need to be said, but Free in this context refers to software freedom, not to price.
People are already able to donate to Mozilla (although not directly to Firefox), if that's what they want to do.
> I collectively paid a lot of money for boxed releases of Red Hat and SuSE back in the day, to support development.
Right, exactly. It's essentially an optional donation, and sadly, few people choose to make optional donations.
> I'm willing to pay for a supported, professional version
Charging for premium support is a different idea.
I'm not opposed to paid Free Software support, in principle I think it's a great idea, but I doubt it would generate much revenue.