By default, Thunderbird shows the short names of newsgroups in the folder pane. If you want to see the full newsgroup names in the folder pane by default, you can edit your configuration to get this.

To do this, you need to get to your preferences editing panel. (On the Mac, you do this by going to Thunderbird > Preferences... in the menu bar. Not sure about other operating systems). Once you get the preferences panel, select the Advanced pane, and then the General tab. Near the bottom right will be a button called Config Editor.... Click it. This will open up about:config editor for Thunderbird.

Scroll down, or use the filter, to find the preference named mail.server.default.abbreviate. If you double click on this row, it'll change the value from true to false. Once you set this value to false, Thunderbird will no longer abbreviate the names of newsgroups in the folder pane.