There are many secret parameters you can use to tweak your Mozilla Firefox or Thunderbird. It is different to access them between Firefox and Thunderbird.
In Firefox, input "about:config" in the address box. Firefox will warn you: "This might void your warranty!". It IS dangerous if you mess up with the parameters. But if you are confident, just click the "I'll be careful, I promise!" button to continue.
You can use the "Filter" box to search the parameter you want. Double click a parameter to change its value. If a parameter has been changed, it is in bold. If you want to reset a changed parameter to its default value, right click it and choose "Reset" from the context menu.
After you finish, just close the tab.
In Thunderbird, the configuration can be accessed from the Options/Preferences dialog. Click the "Advanced" icon in the dialog. Then select the "General" tab. Click the last button "Config Editor..." and the configuration dialog will pop up. It also warns you "This might void your warranty!". And you click the "I'll be careful, I promise!" button to continue. The rest are same as in Firefox.