I think this behavior should be linked with the "Make new tabs have the current focus when opened" option. So if that is not set, and the new tab does not appear, then Firefox should not load. In the devs defense, the inconsistent behaviour between startup and "restore all", and "open in new tab without focus" is easy to miss.

EDIT: I would agree with you in your assessment of the default behavior however, if it were the case, that most people only have a few tabs open normally.