Comment 1 for bug 397121

My experimental service-split branch (which will be the basis for Gwibber 2.0) caches the messages and keeps them around so that the message history goes back as far as the program has been running.

For now, if you want to increase it to more than 20 messages, you can use this hack:
https://answers.edge.launchpad.net/gwibber/+question/56833