The Italian future/conditional tense is formed like many other Romance languages, that is infinitive + habeo/habebam (except Italian uses L habuit for conditional). The F/C stem for most words are the infinitive form, and some words have some sound changes (avere → avr-, volere → vorr- etc.), but one basic word essere has a vastly different stem from the infinitive, sar-. This is also the same in French (être → ser-) but not Iberian (ES/PT ser → ser-).
A similar thing happens also in Catalan: the infinitive is "ésser", but future is "seré", etc. and conditional is "seria", etc.– Charo ♦Feb 25, 2019 at 16:46
@Charo I've always thought Catalan is an Iberian language (groups with ES/PT/Galician).– iBugFeb 26, 2019 at 5:54
@iBug Catalan is (one of) the modern descendent of the "langue d'oc" or occitan, also known as Provençal, who was spread from Piedmont to modern Cataluña and is most famous for its medieval literature. I'm sure Catalan has picked up a few of Iberian features along the centuries, just due to its geographical location, but that's not its origin.– Denis Nardin ♦Feb 26, 2019 at 6:27
From Alkire, T., & Rosen, C. (2010). Romance languages: A historical introduction. published by Cambridge University Press, section 7.8.1 The main Romance future:
The future stem of ‘be’ is a special case. Forms like Popular Latin *ESSER - AT, instead of syncopating, lose their initial syllable to give Italian sarà and French sera (Spanish será could be from *ESSER - AT, but more plausibly derives from *SEDER - AT, given the evidence for syncretism). This apheresis, somewhat unusual in the history of Romance, was probably fostered by the present forms of ‘be’ that begin with [s]. The [a] in the Italian stem has been ascribed to analogy with farà ‘will do’, darà ‘will give’, and starà ‘will stay’, which in turn retain their [a] on the model of the monosyllabic present indicatives fa, dà, sta with stressed [a]. The three languages differ in the proximity of the future stem to the infinitive. They are identical in Spanish (ser ~ ser-), while in Italian and French the future stem has strayed away from the infinitives essere and être. Être, from Old French estre, results from the syncope of Popular Latin essere followed by [t] epenthesis. It had its own future estra ‘will be’ competing in Old French with sera and with iert/ert.
To summarize: regular sound changes would have produced a syncopated form **esr- > ?**err- (cfr. with vedr- for the verb vedere), but analogy with the forms of the present seems to have pushed for the loss of the initial vowel instead. The weird choice of the root vowel in Italian (compared to French) seems to have been by analogic pressure with the verb family fare/stare/dare which had a strong influence on the evolution of the Italian verbal system (compare with the effect they had on the present tense of sapere).
Martin Maiden, in his Linguistic History of Italy concurs with this assessment (section 8.8.5):
it is likely that sarò, the future of essere, is modelled on these verbs [fare/stare/dare].
Rohlfs, in his Grammatica Storica dell'Italiano e dei suoi dialetti is a bit more enigmatic, but fundamentally in agreement (paragraph 587):
Il fiorentino sarò è fatto su darò, starò, farò, dove per ovvi motivi a non è passato a e.
The Florentine sarò is modeled on darò, starò, farò, where for obvious reasons a has not changed to e.