The EFAIL vulnerability is not as severe as it might sound because a key requirement is that an attacker already have access to the encrypted messages. If you used the tips in our security recommendations for PowerPC OS X to improve the security of your computer and your network connection, the odds of this occurring are not zero because the attacker may have already collected them in the past through other means, but are likely to be fairly low with the holes that remain. The risk can be mitigated further by disabling HTML rendering of E-mail (that means all E-mail, however, which might be a dealbreaker), and/or disabling automatic decryption of such messages (for example, I already cut and paste encrypted messages I receive into GPG directly in a Terminal window; my E-mail client never decrypts them automatically). A tool like Little Snitch could also be employed to block unexpected accesses to external servers, though this requires you to know what kinds of access would be unexpected for such messages.
Even with these recommendations, however, there may be other potential edge cases such that until someone(tm) updates Thunderbird or another mailer on Power Macs, secure encrypted mail on our systems should be handled with extreme caution and treated as if it were potentially exposed. If you require this kind of security from your E-mail and you must use a Power Mac, you're probably better off finding a webmail service with appropriate security and using TenFourFox (the webmail service then handles this), or building and using an E-mail client on some other system that is more up to date that you can access remotely and securely (which is what I do myself).