That estimate seems pretty far off.Well, it may well be 50% off, or even 80% off, but it does give you an order of magnitude. 100% correct numbers aren't really material: this is a really good deal for Google.
You are right that not all their revenue comes from search, and I should have thought of that. 66% of their revenue is from Google websites[1]. They don't break that down, but I'd suspect 90% is search (Youtube is probably a reasonable contributor but search ads are very expensive, and Google gets pretty good fill rates on them).
Mobile search is growing quickly, but they still "only" made $2.5 billion over the last 12 months from it (and also I presume this deal covers the Firefox mobile browser, which might not be popular now but Mozilla has been working hard on).
But yes, 25% market share is probably an upper bound.
I think a large number of FF users would use Google search even if that required changing the default.
It is possible that Mozilla could do a deal to change the search engine used on existing installs. I would hope they wouldn't do that, but it is something another search engine would probably have wanted to explore.
Also, about:home is the default home page on Firefox, and currently defaults to a Mozilla branded Google search page. That could be changed on existing browsers for sure.
All up, I agree that the $7.2 billion number is probably high, but it seems likely it would be more than $1-2 billion.
[1] http://investor.google.com/documents/20100331_google_10Q.htm...
[2] http://searchengineland.com/will-google-see-6-25-billion-in-...