Several steps: 1. go to preferences>general and scoll to bottom of right pane and select 'config editor' 2. locate this variable: toolkit.legacyUserProfileCustomizations.stylesheets and set to true. 3. in profiles, create a folder called chrome. 4. in that folder, create a file called userChrome.css (case sensitive) 5. in that file, place this text (I am also attaching a graphic of that, as my insertion of CSS doesn't always work.)

/* Change font-size of email list, folder list and task list */ .tree-rows {

   font-size: 12px !important;

}

6. Close and restart TB.

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