Mozilla engineering manager Josh Aas proposed ending support for OS X Leopard in six months. It lets Mozilla address any final bug or security fixes and gives Leopard owners time to upgrade if they want. Leopard owners don’t have to upgrade as the current version of Firefox will continue to work. They just won’t receive any supplemental security fixes or enhancements to the browser.
While Hodgkins doesn’t mention this, dropping Leopard will mean PPC-based Mac users will be left behind as well.
Update: It seems Firefox dropped PPC support a while back. Carry on.