Point me to a good PSP firmware resource

I just snagged a used one in trade for warhammer models - he said the firmware is an older one that can run homebrew stuff and that I should do my research before upgrading it. My google-fu appears to be weak on this topic because I keep finding a lot of outdated info or sites that want me to pay to access information. I figure if there’s a place where I can get the info I need for free, someone here can point me to it.