Animations are not important enough to justify. Not wanting to serve all your content from a single domain is not enough to justify. These are excuses, not technical reasons.
They're short cuts you might take to get a product out and get early feedback, the same sort of shortcuts most of us have taken when pushed to release something sooner. This is a product in Beta, not the finish article.
Even if everything work in theory just dropping multi-browser testing would save time. If their aim is a beta product for early user testing, I really don't think what they've done is unreasonable.
It's not like the application is completely broken in Firefox with the fixes mentioned. Bypassing CSP for your own domains feels like a hack that you shouldn't be able to do.
Remember with this sort of product the questions they have are "is this interesting?", "is this useful?", "do people like what this does?".
Can we get this running on Firefox simply isn't an interesting question to them - they know the answer, it's yes if they throw resource at it, but for a new product team with questions over whether this product should even exist that's not a priority.
Remember, this is Google who have a long history of canning this sort of project. If I were the company who produced Wave, I'd probably look at how I could reduce the cost and time to feedback for my next new thing too.
As an aside, I've tried Inbox and, to me at least, it's the next Wave, not the next Gmail.
And because of that there is the risk of reputational damage around one of their core products. Working in tech you understand what a Beta really means and you can make that call on an informed basis but that's not true of a typical G-mail user today.
Lazy or evil, pick one.