There are currently two ways to give support on Twitter. The first way is to answer questions with your personal account. This is probably easiest if you just want to help on Twitter every now and then.
If you feel ready to move up to the big leagues and plan on spending some significant time on Twitter support, you should apply for using the official @FirefoxHelp account.
Tip: If you can't help someone in 2-3 tweets, it's usually better to tell them about SUMO, where we can solve even the most complicated problem. A snippet for that would be: Sorry, can't solve this on Twitter. Please ask about it on http://bit.ly/98fQFU People there are ready to help you #FxHelp
We use CoTweet to manage the official @FirefoxHelp Twitter account, which is a web app designed for collaborative team use and a bit of social media overload. If you feel like you're ready to take it on please ask Topal in #sumo on IRC to grant you access.
You can always find us on IRC in #sumo for questions you might have.