i'm on firmware 3.95 (which came with crisis core) and i downloaded a torrent from demonoid which is firefly in mp4 format. it won't fucking play though! it's in my video folder. here is an example:
File size is 190mb
rssolution is 320X176
video codec mpeg-4 484kbps
audio codec aac 127kbps
sampling frequency 44.100 khz

i've tried moving it and re-naming it etc but to no avail
any help would be greatly appreciated.

I take it that you've tried putting the videos in "ms:\VIDEO\", but have you tried:
"ms:\MP_ROOT\100MNV01\" (for non AVC) and rename them to "M4V#####.MP4"
"ms:\MP_ROOT\100ANV01\" (for AVC) and rename them to "MAQ#####.MP4" ?

i have tried this yes but it doesn't work. when in video it says they can't be played, when it's in those other folders it's nowhere to be found

Some Mp4 videos cant be played on psp. Next time your best off finding a download that says its for PSP.