I don't think it is impossible for the French to win a limited war, but not with their OTL line up. They'd need a dramatically different army doctrine for one, defensively oriented, artillery and firepower focused, full usage of fortifications, better usage of reserves, so on and so forth. Dug in along the border in the fortifications they'd make the cost of a German attack ultimately too bloody to countenance. Their navy would have needed to make some different decisions as well, but it isn't impossible for a better organized and more efficient navy to exist in 1914, more battleships and more lighter units and better bases - presuming international politics are roughly the same, just with only the French and Germans fighting, the majority of the German fleet will continue to be trapped in the North Sea and not able to do much. With only limited sections of the German fleet to engage, the French could prey on German colonies to at least some extent in the Pacific, West Africa (especially good if the French build the Trans-Saharan railroad and can pump troops and supplies into the region by land as well as sea), perhaps Namibia?, as well as hurting German commerce. That would probably put the French into a strong negotiating position at the end of the war, occupying German colonies and with a defensive line that would be too hard and too costly for the Germans to break.
If we wanted to push it further I suppose we could magically give the French the Levasseur project working out magically, but I'm not sure that even tanks would be able to survive on an offense when the front lines are so incredibly densely packed, and they would require huge numbers to make the decisive impact needed. Otherwise they'll just get deployed, have some effect, Germans develop their response, stalemate returns. In general too, the last thing we want is the French launching assaults either - we want them sitting in their forts with machine guns and artillery.
On the other hand even under this, with the most optimal changes made to the French armed forces, they still can't "win" a full scale war in Europe, they can just make it so that neither side can attack in Europe and France can win in the colonies. Nobody would be satisfied with the outcome, but it does provide the potential for a limited French victory.
But using the OTL line up the French would be outnumbered and outgunned, with unfortunate results almost certainly resulting in a loss.