Learn French: Le rideau va tomber dans dix minutes. - The curtain is going to fall in ten minutes.

Usually, yes! But when talking about the future, dans followed by a length of time means that an event will happen in that amount of time from now. So dans dix minutes simply means ten minutes from right now.

Could I say en dix minutes instead?

That would completely change the meaning. Dans dix minutes tells us when the event will start (ten minutes from now). En dix minutes tells us how long the action itself takes to finish. Saying en dix minutes would mean the curtain falls so slowly that it takes ten whole minutes to hit the stage!

Why use va tomber instead of the regular future tense?

Va tomber is the near future tense, built by combining the present tense of aller (to go) with the infinitive tomber (to fall). It translates perfectly to is going to fall. French speakers use this structure constantly for events happening very soon, which fits perfectly with a short ten-minute countdown.

Is tomber the normal word to use for a curtain closing?

In a theater setting, yes! Just like in English where the curtain falls at the end of a performance, the French say le rideau tombe. If you are talking about drawing the curtains at home in your living room, you would use fermer (to close) or tirer (to pull) instead.