[outadoc]

The exclamation mark is only shown when you're logged in but got logged out because of a problem. You can just remove your account whenever you want.

[giggles_giggles]

You can also self host the upstream service if you want there features that come with logging in but don't want Mozilla to have the data:

https://blog.mozilla.org/services/2014/05/08/firefox-account...

[bloopernova]

I feel like this should be more widely known. I imagine there's a fair few large corporate IT departments that would love to host their own sync service.