As far as I know, there will not be another version of the Frontier kernel for the Mac, so if you want to keep running the software, you can't upgrade to Mavericks.
Frontier is licensed under the GPL. I put a snapshot of the code into a Github repository. It would be great if someone ported it to the new operating system, or to Linux, where it will likely be more stable. It might even run faster!
I'm still using it on my desktop iMac (running the last version of the OS before Mavericks) and on my Windows servers, where it runs fine. I also have a PowerPC-based Mac that will never be upgraded. I've learned from experience with past platforms that it's a good idea to keep an older machine around. For example, I've wished, when working on Fargo that I could launch MORE to look at prior art. Instead I had to rely on memory.
One approach to using Frontier/OPML Editor on the Mac would be to install a Windows emulator and run it there. I run it that way on my Mac for testing releases and it seems to work quite well.
The first version of Frontier shipped on the Mac in 1992. That's a 21-year run. Pretty good!
Lots of things happened first in Frontier. Blogging tools, RSS aggregators, podcasting, XML-RPC and of course OPML. Manila and Radio UserLand.
It's a great environment, with integration between the database and the scripting language that has yet to be matched anywhere. Its editor both for code and data is an outliner, another coding breakthrough.
This is an about page for the first version to be released for free on the web, in 1996. It gives some idea of the kind of foundation we built on.
Also there's a Frontier-User mail list on Google Groups, if people want to communicate.