I disabled the Avast Shields and the capture worked. With Shields on again it didn't. I then turned off Outpost firewall, just using Windows Firewall and it captured OK.
Your suggestion of clicking allow in the rules in my screenshot changed the word to block and changed the colour of the blob in my first screenshot to red, presumably meaning it is now blocked. Still no capture. I tried deleting these rules but as I launched Screenshotcaptor again, Outpost just prompted for action again, remaking the rules, though not both initially. So short of switching off Outpost and using the Windows Firewall, I really don't know.
Amendment: It seems to be an issue with capturing images from Firefox. Chrome and IE8 are both OK.