I actually don't care that Google is machine-reading email to serve ads (see http://www.maxmasnick.com/2012/02/12/gmail_paranoia/), but I do care about polluting the interface with ads that look like email for the same reason I don't like the new compose interface. Both make the interface worse, and email is bad enough as it is without bad UI.
Fastmail isn't even under the control of your local firewall.
Then turn that function off [1]. There's no more security threat from well-known browser plugins from Mozilla's site than well-known packages from your OS's apt repo. And both are open source.
Why would I be 'uncomfortable' with ads? Sometimes I feel like the issue with ads in Google/other services falls victim to the 'Nickelback Effect', where a few other very vocal people hate the subject at hand and it spreads virally to the point where people can't describe why they hated the subject in the first place.
I bet not.
Google could easily decide to serve ads to paying customers, and it seems likely that they've left this option open to exercise later.
The fact that they apparently "do not rely on ads" is irrelevant.
There are plenty of existent services that have revenue from customers, but still serve ads (visible examples include: pay-TV, public transport)