Go to Options Type config editor into the search box the result will be a single config editor button. Select it. In the config editor dialog type layout.css.devPixelsPerPx in the search box Right-click the preference layout.css.devPixelsPerPx, Select Modify, and set it to 1.25, OK to close Options. and then the X to close the about:config dialog. Restart Thunderbird.

Note that setting it to 1.25 makes the fonts 125% of what they were. Setting to 2 will double the size and may make Thunderbird unusable because of huge fonts, so use care and a lot of discretion when modifying the setting. Small changes until you get exactly what you want is the best way to manage this.