That will make Firefox a very attractive browser to people who want to block as many ads as possible, and they will then install an add-on which also blocks the ads that Firefox whitelists, perhaps using Firefox's own whitelist as a source if it's easily available. Then if Firefox tries to pull a Google and be evil and lock down their add-on API so that ad blocker add-ons can no longer be made, these people will just switch to a fork with no such limitations. And in the first place I don't think Firefox has enough usage for them to be able to charge a "fortune" for such an ad blocking exemption in the first place.
By "EU and US govs mandating it as the only default browser" I assume you mean for government employees and not for the entire populace (which would be straight up despotic), but either way it would probably be unenforceable.