yeah, you'll need to define 'connections'
even failed remote login attempts to the firewall are a connection (ack)
if this is more of a "once XYZ interface hits XX Mbps" and if it is safe to assume you have a server/pc on the private side of your network, then fetch the free version of this: http://www.manageengine.com/network-monitoring/ the free version is full featured and does up to 10 devices, defined as IPs, so the single management IP of your device would only count as one device, regardless of the count of interfaces/subinterfaces/vlans. do snmp polling of your interfaces and set it to email/page/sms/log based on a given interface or vlan hitting X Kbps/Mbps, etc.
note, i'm hoping to get opmanager running against pf, haven't yet, but i use it in other sites and against other snmp capable hardware and software firewalls/routers.
if you need to know when an IP behind the firewall is having a series of connections being passed, at a more granular level than just interface or subinterface, then flows (netflows/sflows) model will work. but that's not free with opmanger. try prtg for that. http://www.paessler.com/tools it's limited to "10 sensors" to remain free, but that includes 'each item monitored" like IPmon now solarwinds, so you can blow through that in one device pretty fast.
both tools support alerting based on triggers.