"Doctor it huts when I do this!"
"Don't do that!"
As marcelloc said, snort is probably filling it up some way. Could be logs, or rules, or who knows what. snort is a large package. Trying to make that, and all its data, fit in a 512MB slice along with the base install of pfSense is probably not going to work out very well.
Grab a 4GB CF. They're cheap, buy a couple spares.