French Cuirassiers only a year before WWI would begin, looking much the same as they did under Napoleon; ca. 1913

And for most of that first year of fighting in WWI, they continued to look like this. French soldiers were massacred because of these dated outfits.

At the outbreak of war the French Army retained the colourful traditional uniforms of the nineteenth century for active service wear. These included conspicuous features such as blue coats and red trousers for the infantry and cavalry. The French cuirassiers wore plumed helmets and breastplates almost unchanged from the Napoleonic period. From 1903 on several attempts had been made to introduce a more practical field dress but these had been opposed by conservative opinion both within the army and amongst the public at large. In particular, the red trousers worn by the infantry became a political debating point. Adolphe Messimy who was briefly Minister of War in 1911-1912 stated that “This stupid blind attachment to the most visible of colours will have cruel consequences”; however, in the following year, one of his successors, Eugène Étienne, declared “Abolish red trousers? Never!”

(Source)