WHAt is a custom firmware

does it mean i could web browse and still access homebrew from my memory stick

a custume firmware is a firmware that lets u do everthing a 1.5 firm does but with web broswing , custume xmb , homebrew
i uses 3.03OE-C but theres 3.4OE-A thats good for psx emulation

Custom firmware basically combines two firmwares together so you can have the functionalities of both. It combines 1.50 and whichever firmware you see in the title of your download. 1.50 allows you to run homebrew and all the other good stuff, while the other firmware gives you the functionalities that it posess' (usually psx emulation, internet, and whatever else was added in)

Just think of it as a mod chip without having to install a chip.

Thats probably the best i can explain it, if you need help finding a good custom firmware...in my opinion 3.03 oe-c is the best, others would say 3.40 oe-a. So just pick whichever of those two you want, theres really not much difference in either of them.