Actually, according to this Macworld article Mozilla will be auto-upgrading people running Firefox 3.6. Such news may not come as a surprise to you, but if you have a PowerPC Mac 3.6 is the very latest version of Firefox that will run on your Mac. By auto-upgrading those machines, Mozilla will effectively break the browser on that platform.
The good news is that there are some alternatives. If you have a PowerPC Mac try out these browsers:
TenFourFox is an unofficial Firefox browser specifically tuned to your machine’s architecture. It is probably your best option as its built from the latest Firefox source. Note that with this project the source has been modified so they cannot legally call it Firefox (even though your machine won’t know the difference). For security reasons TenFourFox comes with plugins disabled. If you wish to enable plugins type about:config in the browser’s address bar, and then change the tenfourfox.plugins.enabled setting to true. Mozilla recommends you use TenFourFox in their MozillaWiki.
El Furbe is also tuned to your PowerPC machine’s architecture, but the latest version available is built from Firefox 4 source.