There are many ways to send Twitter messages.
On the one hand you can send your messages with the Twitter HP or on the other hand you can send twitter messages on your mobile phone. For that you do not need any other apps, because you can use your browser on your mobile phone. Only enter m.twitter.com and start posting.
An other possibility to send twitter messages is an opera widget. I'm using the "Twitter Opera widget". It's small and offers all options, you also have on the Twitter HP.
For Firefox, there are also some Add-ons, but I didn't test them so far.