Most words ending in "ability" formed in English are first attested after corresponding words ending in -able.
Note that this is not simply -able + -ity: in English it is a parallel formation, not a compound suffix: there is a sound and spelling change. Compare -ably (compound suffix) and -ification (parallel suffix).
Some words ending in "ability" are alterations of words of French origin ending in "abilité" or, possibly, Latin words ending in "abilitas".
► <a class='CategoryTreeLabel CategoryTreeLabelNs14 CategoryTreeLabelCategory' href='/wiki/Category:English_words_suffixed_with_-ability'>English words suffixed with -ability</a>