There does not seem to be any simple way to increase the display font size in the Thunderbird Message List Pane. There is an archived discussion of this issue at [https://support.mozilla.org/en-US/questions/1267898 Thunderbird font size] but the writer refers to an “Open Finder” button which does not exist on the Troubleshooting Information tab (there are two Open Folder buttons). Also, his suggestion requires replacing a key file in the Thunderbird profile folder, which could cause problems. Any ideas or suggestions will be much appreciated.