Beta and earlier users tend to be different in a lot of ways from regular Firefox users. Soft launching surely helps the download servers, but a large point is to watch for new crashes, so that any emergency fixes can be put in before unthrottling the release.

Past issues include antiviruses that crash Firefox because of their invasive monitoring. While ideally these things would be caught by QA or early adopter users, there's no way to test all possible combinations of software.