Pronunciation of the future tense in French
There are three main issues with the pronunciation of the future tense:
The first of these points means that in normal, non-emphatic speech, many (typically northern French) speakers pronounce donnerai as donn'rai (2 syllables) rather than donn-e-rai (3 syllables). And even for these speakers that would pronounce this as three syllables (or any speaker giving it an emphatic pronunciation), the middle vowel would be a schwa [ə] (or that's speaker's pronunciation, often [ø]) rather than the close [e] vowel of the infinitive. So in the spoken form, it's not quite true that these verbs form their future tense with the 'infinitive plus future tense ending' as the written form suggests. If you imagine the (spoken) future tense of -er verbs as consisting of the singular present tense form plus -rai, -ras etc, this actually turns out to be a simpler rule, since it also accounts for j'appellerai and je mènerai, as well as accounting for the 2-syllable pronunciation of je donn(e)rai.
Other forms with pronunciation difficulties
Two forms which are often mispronounced by learners are the future tense forms of the verbs faire and voir. The first of these has forms ferai, feras etc. The e vowel of these is pronounced as a schwa (like the vowel of le).
On the other hand, in the future form of voir, (je) verrai the e is pronounced as a close [e] sound (as though written é). In fact, this is generally the case in French for e occurring before a double r or double s in the written form.