OS X Mavericks is available today as a free upgrade. People at Mozilla are probably asking if it is safe to upgrade: will it affect my ability to build Firefox.

A few people (myself included) have been running OS X Mavericks developer previews for a few months. I believe all the Firefox build system issues have been worked out for at least a few weeks now. So, upgrading to OS X Mavericks should not impact your ability to build a stock Firefox configuration from mozilla-central.

There might still be some non-default features and code paths that need fixed to work with Mavericks. Support for the OS X 10.9 SDK might also be problematic. In addition, if you build older trees such as Aurora and Beta, you may run into issues building on Mavericks because those trees may not have all required fixes uplifted.

While I won't encourage you to upgrade, I will say that Mavericks should build Firefox without any issue. And since the number of Mavericks users will only increase in the days ahead, it should be safe to assume that regressions will be promptly fixed.

If you run into any issues with Firefox and Mavericks, bug 883824 is the master tracking bug. Bug 894090 tracks build system issues.