It would be helpful to understand what is going on if you could post this info: In Thunderbird

-   Help > Troubleshooting Information
-   scroll down to 'Important Modified Preferences'
-   Copy all lines that start with 'font.' and paste info into this bug report.

I suspect you have a load of minim font sizes set much larger than the fixed and variable, so it's overriding everything. I need this info both to help you get things sorted and also because some people think it's a bug.

kari12, please help us with feedback about the following....

We have determined that some users somehow have a non-default value for hidden preferences ...

Check the Config Editor for this settings:

• Settings > General
• scroll to bottom and click on 'Config Editor' button

It opens in a new tab

• In top search type: pixels
• look for this line : layout.css.devPixelsPerPx
• default is a negative number -1.0

It should not be in bold text, if bold text then it has been altered.

• If you have anything else in that setting use the arrow icon on far right - click on it and the default is reset.

Another value to check is mail.uifontsize, what value do you see?

Please post your results