When I started Thunderbird, I found I had no mail in any of my folders. My default folder languages had also changed to Arabic. Middle-clicking on links launched a new, yet empty, tab. I decided to move the compreg.dat file in the.mozilla/firefox/default.XXX directory. After restarting Firefox, it too worked fine. After moving the.dat file, a restarted Thunderbird performed as it should have. It also couldn’t use my arrow keys to navigate.”]
Source: https://taosecurity.blogspot.com/2004/11/fixing-weird-behavior-after.html