I doubt it has anything to do with your firewall. It happens to me sometimes also.
Sometimes, servers do not show up in the list, but you can still connect to them. Instead of looking in "internet" tab, look in "history" (or "friends" tab if you have friends playing SH). Servers might show up here, and you can connect to most of them here (the FM servers, the SFC servers, and sometimes Stryker).
There maybe is, but I don' know any alternative way. =( This bug among other will most likely never be fixed since FM are working on other games now.
I hope you will be able to see the servers soon, when the bug happened to me it didn't last for more than a day.
And if you have no server in history, maybe you have friends playing SH? Well I'm not sure you do because you didn't mention it... I recommend that you join the group Shattered Horizon 2012 on Steam, and when someone from the group is playing SH, add him to your friend list and join the game through the "friends" tab in Shattered Horizon server list.