Mozilla is mulling the end of Firefox support for Mac owners running Leopard, the Apple operating system released four years ago, developer discussions show.

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In a message Wednesday to a planning thread, Mozilla engineering manager Josh Aas proposed that the company pull the plug on Mac OS X 10.5 in six months.

“Maintaining Mac OS X 10.5 support consumes a non-trivial portion of the resources we have available for Mac OS X development,” Aas said. “Not maintaining Mac OS X 10.5 support will allow us to devote more resources to the product as used by the majority of our Mac OS X users, those on Mac OS X 10.6 and 10.7.”

Citing Mozilla’s own data, Aas said that 24% of active Mac Firefox users currently run the browser on machines powered by Mac OS X 10.5, aka Leopard. By June 2012, when Aas suggested support stop, Leopard users should comprise only about 13% of Mac Firefox users.

Aas also noted that Apple itself has halted support for Leopard; it released the last security update for the 2007 operating system last June, several weeks before the launch of Mac OS X 10.7, better known as Lion.

The last time Apple updated its own Safari browser for Leopard was July 20, when it released Safari 5.0.6.

Apple typically stops issuing security updates for an older edition of Mac OS X when there are two successors available.

If Aas’ plan is adopted, Firefox 13, now set to ship June 5, would be the last to support Leopard.

Although there were some dissenting voices in the discussion thread Aas kicked off, none objected to the eventual demise of Firefox support for Leopard, instead wondering when would be the best time to retire Mac OS X 10.5.

Some Leopard users have already been left behind by Mozilla. The open-source developer dropped support for PowerPC-equipped Macs earlier this year. Leopard is the newest Apple OS that runs on PowerPC, as opposed to Intel-based Macs.

Mozilla ditched Leopard’s ancestor, Mac OS X 10.4, or Tiger, in early 2010 after the launch of Firefox 3.6.

Firefox fans have other options in the face of the looming retirement.

A group of developers calling themselves TenFourFox have created a browser built with Mozilla’s code but fine-tuned for PowerPC Macs running Tiger or Leopard.

Camino, also built atop Mozilla’s code, is another option: Camino runs on Tiger and later, and on both Intel and PowerPC Macs.

Both TenFourFox and Camino are free downloads.