Answer: Much like verb forms such as "wanteth" and "saith" in English, the future subjunctive in Spanish is all but obsolete. You're extremely unlikely to hear it used in everyday speech; the only times you're likely to come across it are in literature, in some legal language, in especially flowery language, and in a few phrases such as "Venga lo que viniere" (come what may) or "Adónde fueres haz lo que vieres" (roughly, when in Rome do what the Romans do). It is fairly common in plays from the Golden Age, so it appears that at one time it was used in both speech and writing. But today it has all but disappeared.
Fortunately, if you ever have the occasion where you need to know the future subjunctive, it's fairly easy to learn — if you already know the r form (the more common form) of the imperfect subjunctive. The a in the imperfect subjunctive ending is replaced by an e, so the future subjunctive forms of hablar, for example, are hablare, hablares, hablare, habláremos, hablareis and hablaren.
Generally speaking, today the present subjunctive is used for both present and future tenses where the subjunctive mood is called for. Thus, in a sentence such as espero que me dé un regalo ("I hope she will give me a present") or no creo que venga ("I don't believe he'll come"), the present subjunctive (dé and venga) is used even though we're talking about an event that might happen in the future.
You have no need to learn the future subjunctive for fully competent use of the language, just as the foreign learner of English typically has no need to learn the verb forms of Shakespeare or the King James Version of the Bible.
Question: So ... when was the future subjunctive used? Was it used only in such expressions as venga lo que viniere? Or could it be used with something like esperaré que viniere or no creeré que viniere?
Answer: Yes, I have found a few examples of such usage, although I can't give you an authoritative answer of how common it was. My reading of some literature also indicates that it was often used in clauses following si (if) and cuando (when), such as in si tuvieres mucho, da con abundancia (if you have much, give generously). In those cases now we'd usually use the present indicative in Spanish.
In current legal usage, where the future subjunctive is most common today, the form is used mostly in cases involving an indefinite person (translated "one who" or "he who") as in "el que hubiere reunido mayoría absoluta de votos será proclamado Presidente de la República" (the one who receives an absolute majority of votes will be proclaimed president of the Republic).