From looking through a few threads on other forums it may be a bug in your Thunderbird profile. If you can be bothered to work through the content of one of the fixes here is a link, comment 9 cured it for that poster. Also a link on how to access your Thunderbird profile.
To find my pref.js I went to click here which told me to:
How to find your profile. Open Thunderbird
Click on the menu button or menu bar.
From the Help menu, click Troubleshooting Information.
In the Application Basics section, Profile Folder, click on Show Folder
The Windows Explorer window will show the name of the profile as well as the path to it.
When I clicked on Show Folder, Windows Explorer opened the folder and I could see my prefs.js which I then rightclicked on and chose Edit (then it opens in Notepad and you can edit it)
You have to close Thunderbird down before you can edit the prefs.js or else your edits won't save.
I suggest you make a copy of prefs.js and re-name the copy by adding .txt to the end so that you can go back to that should you mess up the original. Also note I have got 'show hidden files and folders ticked' so you might have to do that as well.