A number of very common verbs use an irregular stem in the futur simple, even though the endings themselves (-ai, -as, -a, -ons, -ez, -ont) stay completely regular. Être becomes ser- (je serai), avoir becomes aur- (j'aurai), aller becomes ir- (j'irai), faire becomes fer- (je ferai), and venir becomes viendr- (je viendrai).
Other frequent irregular stems include pouvoir → pourr- (je pourrai), vouloir → voudr- (je voudrai), devoir → devr- (je devrai), voir → verr- (je verrai), and envoyer → enverr- (j'enverrai). These stems are worth memorizing directly, since they don't follow any single predictable rule — but once learned, the same endings apply to every verb without exception.