First off, why the -k? That really should only be used on smaller applications, such as Lilo and cron. With the actual applications, the whole point is to get the source and compile it to allow it to be optimized to your processor. So:
ACCEPT_KEYWORDS=~x86 emerge mozilla-thunderbird
Will compile the latest version of the application for your hardware. SO...
i386 OR i586 is irrelevant, as it's i whatever 86 your processor is
This assumes you are on Intel architecture, which includes Pentium and Athlon processors.