Hello out there!
I am using an XL2 with a Macbook OS 10.4.11 connected through Firewire. The problem is iMovie only recognizes that the camera is connected intermittently. Once established it stays but initially its only sometimes. The firewire port is fine and works well with my XTHD and I used another cable with the same problem. I have shut down the computer and the camera several times and tried connecting and disconnecting in all different sequences. Has anyone else had this problem? Any suggestions would be appreciated.

Are you connected directly to your Mac via Firewire, or is it running through an external Firewire hard drive's port? Canon cameras are well known to not "play nice" with Firewire especially if there are other Firewire drives in the chain.