"Explain." OK.

1) No Firefox fork is doing large-scale engineering work on the browser. They're all small teams whose main contribution is things like different configuration settings. If Mozilla dies, these forks will all die too.

2) Some forks consider it a feature that they keep old, insecure APIs Firefox itself abandoned because they could not be secured (NPAPI, XUL, etc.) Mozilla could not secure these APIs; forks aren't going to be able to. Most don't bother trying.

3) You're still running on Mozilla code, so your trust model still includes Mozilla. Now it just also includes a third party. Do you trust them? Why?