Either click one message in the Trash folder or on an empty section below them, press Ctrl+A and press Delete on your keyboard, or right-click on the Trash folder on the left and select Empty Trash.

And to add to Tonnes's suggestions…

If your 30 messages are in a contiguous block, click the first one to select it, scroll down to the last, hold down shift and click this last message. All messages between the first and last (inclusive) will now be selected.

Tonnes told you how to delete all the messages; I am assuming that you want to delete just 30 specific messages and leave others in place. I don't understand why people want to store things in the Trash, but what the hey.

If your messages cannot be coerced into a single contiguous block, you can toggle individual messages between selected and unselected by holding down ctrl when clicking on them. So you might select 31 messages in a block and then de-select the one message in the middle of the 31 that you want to keep.

Sorting by date, sender, subject etc are all useful tricks to get your 30 all in one contiguous block. The QuickFilter bar is also useful in that it can be used to hide the messages you don't want to delete, then use the ctrl+a on what's left behind.

But if you are considering deleting some but not all the messages in Trash, please have a hard think about whether or not you could find a better place for those you want to keep.

Personally, I aim to keep my Inbox, Sent, Trash and Junk folders empty. Valuable or useful messages are filed into specific folders.

Oops, seems I overlooked the 30 messages selection, sorry. ;)

The ways and shortcuts to select and delete are also described in the Keyboard shortcuts - perform common Thunderbird tasks quickly article.