Why that? AFAIK, they were not allowed to use the name firefox since their version of firefox contains or used to contain unofficial patches and mozilla forbids the use of the name "firefox" in that case.
Renaming was one of the options, dropping the patches the other.
Some do, but they have them Mozilla-approved. Vendor-specific patches are allowed in a product called "Firefox", if Mozilla approves the patches before release. I believe Red Hat is one vendor that does that. Debian won't agree to include a package that needs third-party approval of modifications, though.
That is the 4th criteria of the Open Source definition at work. I think it is a good rule, and makes sense. Mozilla is sharing their code, but doesn't want their name to be used for code they didn't approve.
Renaming was one of the options, dropping the patches the other.