Mozilla will probably cut off Firefox's PowerPC support past v3.6, according to the company's director for the browser, Mike Beltzner. "I am gathering data on the number of PPC users we have, but the likely outcome is that we will not be supporting PPC [PowerPC] for Firefox 4," he writes in a mailing list post. The root of the problem is said to be two new features in Firefox 4, beginning with out-of-process plugins, which can prevent the program as a whole from crashing when something like Flash becomes unstable. Also mentioned is a just-in-time JavaScript compiler, borrowing technology used in Chrome and Safari.
"I believe that Firefox 4 should be relatively faster and superior to previous versions -- so to be better than Firefox 3.6, two of the significant mechanisms for performance improvement will not be available for PPC based computers, and so we are not planning on extending Firefox 4 support to those architectures," Beltzner elaborates.