How are these 3 things related? Is firecore the company that created seas0npass and aTV Flash? aTV Flash seems like an ATV addon similiar to XBMC? And you must use seas0pass to jailbreak ATV before aTV Flash can be installed?
5.3 is seas0npass tethered boot right now, if I buy aTV Flash which supports 5.3, is it still tethered boot?
basically correct.
SeasonPass is the free jail breaking app that Is based on jailbreaks developed outside FireCore but bundled up to try and make the jail breaking process easier. It is not required to buy ATV Flash to use SeasonPass.
ATV Flash can only be installed on a jail broken system and includes inFuse which is an alternative to XBMC that many prefer to use. It also includes some additional apps and Also makes it easier to install some non-FireCore apps such as XBMC. ATV Flash is only tested against systems jail broken using SeasonPass although it normally installs and works fine on systems jail broken with other tools.
5.3 is seas0npass tethered boot right now, if I buy aTV Flash which supports 5.3, is it still tethered boot?
Yes. There is only a tethered jailbreak for 5.3.
This is unlikely to change now as 5.4 (which is based on iOS7) is not that far off and any jail breaking efforts are almost certainly going to be geared towards that release.