Josh Aas, Firefox developer, says Apple provided preliminary development as a basis for porting the browser onto the new platform. He says Apple wanted Firefox running on an Intel Mac as a demonstration of what it takes to port a complex application. After the demo Apple sent the patches to the Firefox team.
Josh first says the patches weren't really helpful but later he claims they were extremely valuable:
“I never tried to get Firefox running on Intel Macs by just applying their patches. For one thing, they were not worried about cross-platform patches or writing the code in such a way that we could actually land it in our [source code] tree," he added. "They just wanted it to run”More details at Softpedia
In any case, these patches were not very helpful, because some of them were rather obsolete, due to the developments Firefox's code had undergone since they had been created.
"However, the Apple patches were extremely valuable because they did a lot of work for us and at least pointed us right to many of the problem areas instead of us having to figure out what we need to do," wrote Aas.