Learn English – the plural of gif

Although the proper Latin plural would be fora, forum has been adopted into the English language--and in most cases follows the rules of English pluralization. Similar changes can be seen with the adoption of other words like octupus (the proper plural would be more like octopedes, but in English we usually say either octopi or octopuses).

The current entry from the Oxford Dictionary says:

The plural of forum is usually spelled forums; the plural fora (as in the original Latin) is chiefly used when talking about a public square in an ancient Roman city.

Since you're talking about online discussion boards, I would use forums instead of fora when you are talking about more than one. An English speaker might understand fora, but it wouldn't come as naturally.

Staff is not different from other collective nouns: It takes a singular verb when it refers to the collection as whole, and a plural verb when it refers to the members of the collection taken individually.
That is for American English; in British English, collective nouns are generally regarded as plural.

See Collective nouns with singular verbs and plural pronouns.