I'd like to propose that we remove support for Mac OS X 10.5 in Firefox 16, which should ship on or near October 2, 2012. To be clear: this is not a decision that has been made, I’m proposing it here in order to get feedback.
First, some facts:
Mac OS X Release Dates) 10.5 was released in October of 2007, 10.6 was released in June of 2009, 10.7 was released in July of 2011. Mac OS X 10.8 will be released during the summer of 2012.
Mac OS X User Breakdown) Mac OS X users make up 5.0% of Firefox 12 (current stable) users as of May 12, 2012. Of those Mac OS X users, 17% are on Mac OS X 10.5, 35% are on Mac OS X 10.6, and 48% are on Mac OS X 10.7.
Trends) Mac OS X 10.5 users have been declining by 1-2% per month (as a share of our total Mac OS X users). This means that when Firefox 16 ships, Mac OS X 10.5 users will likely make up less than 10% of Mac OS X users.
Apple releases new versions of its operating systems relatively quickly and each new version contains significant changes that we must adapt to. This requires resources, and with limited resources this sometimes means we have to make tough decisions about where to invest.
We still have significant integration work to do for Mac OS X 10.7 and we’re getting more for Mac OS X 10.8. Currently we have to go back and make sure we didn’t cause problems on Mac OS X 10.5 every time we do integration work for newer OS versions. In addition to the core Mac OS X platform layer (Cocoa widgets), subsystems such as plugins and graphics are affected by the Mac OS X 10.5 requirement.
Build, release, and testing infrastructure resources are also consumed by the Mac OS X 10.5 requirement.
There are already some significant ways in which Firefox on Mac OS X 10.5 has fallen behind Firefox on newer versions of Mac OS X. Accelerated compositing, WebGL, and out-of-process plugins are not available on Mac OS X 10.5.
Google has already announced their plans to drop Chrome’s support for Mac OS X 10.5 . Their timeline is similar to what I'm proposing for Firefox.
Finally, Apple has already stopped supporting Mac OS X 10.5, and Adobe will likely stop shipping security updates for Flash on Mac OS X 10.5 soon. While Apple does not officially drop support for older OS versions, they have stopped shipping security updates and updating applications like Safari.