I'd also predict an SNP gain in Falkirk if there were to be a byelection. The biggest consolation for Labour though is that the SNP may be a bit short on decent potential candidates who aren't already at Holyrood.
That, and that the SNP invariably do better in these sorts of Labour/SNP battlegrounds in Westminster by-elections than in Wesminster general elections, and that anti-SNP protest voting for Labour doesn't seem to be in fashion yet.
And since Labour managed to create a problem for themselves here with Dennis Canavan, the voters of Falkirk have rather got into the habit of not voting Labour other than at general election times - Labour have never won the equivalent Scottish parliament seat in 4 attempts and the SNP won the popular vote in Falkirk council last year (albeit narrowly and taking one less seat than Labour).
"God knows I'm no Tory, and I never set eyes on a Whig yet without feeling the need of a bath..."
I think UKIP would beat both the Tories and LDs in a Falkirk by-election, possibly saving their deposit as well.
The stats from both Falkirk SP seats in 2011 showed getting around 200 votes in each constituency, approximately 0.8% of the vote. The Lib Dems were up over 2% so not out of the realms of possibility to overtake them depending on the precise nature of the contest, but the Tories would likely be closer to 8% as there are some blue pockets around the town. I'd expect the SNP and Labour to carve up 85% if not 90% between them though at any time.
I'd guess that the Conservatives would beat UKIP in a Falkirk byelection by a good ratio (at least 2 to 1) and would also hold their deposit. For the Lib Dems it would clearly be tougher but I think we might beat them.