On the 29th of Feb 1940 (I guess it was a leap year), France promised Finland 50,000 troops and 100 bombers if Finland refused to surrender.
IOTL, Norway and Sweden did not give permission to the Wallies to bring forces in, so the promise was meaningless.
WI Norway and Sweden allowed for the transfer of men? Perhaps the POD is that Germany attacked France in Nov 1939 and were bogged down. This as a result gives the Wallies free reign in Scandanavia and the Russians, perhaps waiting for the German collapse in coming months, put more men in Poland, effectively sapping Russian strength.
Further, the Finns have more ammo, as Norway and Sweden open themselves up to the Wallies, allowing the Finns to hold out longer at the Mannerheim Line.
The result is that by the time the Russians establish bridgeheads near Vipuri by crossing the frozen waters (delayed 2 weeks by increased Finn ammo and less Russian forces), the spring thaw makes these positions collapse.
At this point the Mannerheim Line collapses, but the second, lesser built up line now has 50,000 French soldiers there to help the Finns. Let's say at this point we are in late March.
What happens next? Feel free to speculate how this may affect the war with Germany, and if the USSR essentially becomes a full-blown co-belligerent with Germany.