Like with a marker pen? ;-) Try this add-on: https://addons.mozilla.org/en-US/thunderbird/addon/highlighter-222207/ It seems rather old but still works for me here in TB38.6.0 Easy way to install: # Go to '''Tools|Add-ons''' in Thunderbird # Type ''highlighter'' into the search box and click it # Select ''Highlighter 0.6.5'' from the list # Click its ''Install'' button # Restart Thunderbird