On 18 February 1853, the Emperor Franz Joseph I of Austria was assassinated by Hungarian nationalist János Libényi, who struck the emperor from behind with a knife between his fourth and fifth rib.
With no children, the emperor, was succeeded by his 21 year old brother, who became Ferdinand II (OTL Maximilian I of Mexico) Ferdinand had hoped for a life in the navy, but had been pushed onto the throne by fate.
Ferdinand would reform not only the navy but also the nation army and although he was interested in the military, he was also aliegned with liberal politics, in OTL, as Emperor of Mexico, he had:
been shocked by the living conditions of the poor in contrast of the upper class, the royal couple began holding parties for the wealthy Mexicans to raise money for poor houses. One of his first acts as Emperor was to restrict working hours and abolish child labour. He cancelled all debts for peasants over a certain amount, restored communal property and forbade all forms of corporal punishment. He also broke the monopoly of stores and decreed that henceforth peasants could no longer be bought and sold for the price of their debt.
To the dismay of his conservative allies, he would bring about several liberal policies such as land reforms, religious freedom, and extending the right to vote beyond the landholding class.
His marriage to Princess Charlotte of Belgium (later Empress Charlotte), the daughter of Leopold I, King of the Belgians and Louise-Marie of France. She was first cousin to both Queen Victoria and Prince Albert, which gave Britain and Austria an unlikely close relationship.
IOTL Ferdinand and Charlotte had no children together, due to him being executed, but if he lives nearly as long as his brother did in OTL. I could imagine him living to nearly 1920s with one or two of their children marrying into the British family, most likely a second son married to Princess Beatrice or a daughter marrying Prince Leopold, Duke of Albany.
I doubt Ferdinand's liberal ideologies would form revolts in the Balkans, if anything, he should be able to strengthen his claim in this region, but if the "Great" war, should come about, I believe his reformed army should be able to defend its rule in the region.
I like this, but I strongly doubt that he would take the regnal name of Ferdinand. Ferdinand I. of Austria, the poor hydrocephalic, epileptic man, was the butt of many jokes and a weak ruler in a absolutistic system. Only five years earlier he had been forced to hand over the government to his nephew in the wave of revolution all over the monarchy.
So Maximilian Ferdinand will very probably rule as Maximilian I. If he had had a strong personal predelection for the name Ferdinand, he could have decided to become Fernando I. of Mexico, where the negative of example of Ferdinand of Austria wouldn't matter.
More general: Little can be as confusing as calling a person "liberal" in the 19th century.
In these times, political liberalism in the Habsburg was almost inevitably connected with the urban bourgeois of Italy, Austria (especially the German-language subjects) and Hungary (ie the Magyar-language subjects). Liberals tended to be strongly against political and secular power of the (RC) church and many were in favor of full emancipation of the Jewish minority.
The tendentially conservative, pro-clerical, rural and aristocratic leadership of the Slavic subjects were the natural opponents of political liberalism.
Some events of the reign of Max1:
Whatever his inclinations are, he will not be able to do beneficial things for Hungary for quite some time. The public backlash against a Magyar killing a Kaiser will be too strong.
In February 1853, FJ has not even met his (and Max's) cousins Helene and Elisabeth yet; the first meeting was in August. Don't know if Max would be bound by a official time of mourning, but chances are good that he will never marry Sephanie of Belgium, but either Néné or Sisi instead. Another candidate would have been Princess Sidonie of Saxony, who was presented to FJ even earlier, but whom he disliked. Can't find for which reasons.
His succession will not change the start of the Crimean war. He might be much more tempted by joining the war on the "liberal", Western European side than his brother was. If Austria actually enters the war against Russia, European history quickly become very different.
No idea how it would turn out militarily.
Regardless of your belief in "butterflies" I am convinced that these changes in Vienna in 1853 will have enough small consequences in Italy (especially in the Lombardo-Venetian Kingdom) that the assassination attempt on Emperor Napoleon III. by Felice Orsini in 1858 will not take place at all or be changed drastically. Nap3 might not become interested in an Italian War at all, or he might even die when his carriage eplodes. The French Civil War between Republicans and Monarchists
might be the defining event of 1858+.
Note that if nothing happens in or concerning France and Italy, then the German Confederation will nothave to mobilize the Federal Army to guard the Rhine against French attacks. The deplorable state of the Prussian milkitary will not become so glaringly obvious and thus the army reform plans will be pursued less urgently. The conflict between the King and the Landtag might not start or be far less severe, and the Prussian ambassador in St.Petersburg, Count Otto von Bismarck, will be never offered premiership as a desperate Hail Mary attempt by the Berlin court.
OTOH, the asassination attempt on Wilhelm I. by Oskar Becker in July 1861 in Baden-Baden is based on his general conviction that the Prussian king is a big hurdle for German unity. Political changes in Austria will not do much to change this. Who knows, perhaps his bullet finds his target (OTL it grazed the king's neck) and we have the ultimate liberalism-wank, with Kaiser Maximilian in Vienna and a young and healthy König Friedrich III. in Berlin.
(Of course, if the two most powerful monarchs in germany are assassinated both within eight years, the reaction might be fierce and decidedly illiberal.)