It is not possible to install Mac OS X 10.6.* (Snow Leopard) on an iMac G5.
Mac OS X 10.5.* (Leopard) was the last version of Mac OS X available for any PowerPC-based Mac. Snow Leopard was Intel-only, although it could run some PowerPC / Mac OS X applications using Rosetta 1.
The original form of https security was found to be less secure than thought. There was a move to upgrade both Web sites and Web browsers to stronger protocols. My understanding is that in the case of the Mac, the change first appeared in a version of Mac OS X 10.11.* (El Capitan), though it may have then been back ported to the latest releases of the immediately preceding operating systems.
None of the versions of Mac OS X that run on PowerPC-based Macs have that change, so you'd have to hope that some developer of a third-party Web browser cared enough to port the security change back that far. It definitely won't be in PowerPC versions of Safari.