From MozillaZine Knowledge Base
If you see a thick gray bar with red text below the status bar of your Mozilla application, it is usually caused by a faulty extension. Sometimes, the gray bar will include red text that provides a clue as to which extension is causing the problem, but usually it just contains a ^.
- Check the list of problematic extensions.
- Look for updates for your extensions. (In Firefox or Thunderbird, choose "Tools -> Add-ons -> Extensions -> Find Updates".) If any updates are found, install them and restart the Mozilla application to see if the the bar is gone.
- Google Web Accelerator or Google Photos Screensaver may cause this problem. If you have them, uninstall or reinstall them. 
- Disable your extensions one by one, starting with the most recently installed. Restart the Mozilla application after every disable. If the gray bar disappears, it's the one you just disabled that's the problem.
- If the problem is in Firefox on Windows, check for a file named m3ffxtbr.manifest in the "chrome" folder of your installation directory (usually "C:\Program Files\Mozilla Firefox\chrome\m3ffxtbr.manifest"). If the file exists, delete it, then restart Firefox. This file is created by the "MyWebSearch Toolbar". This file is automatically removed by Firefox 220.127.116.11 and later releases. That file can also be named a2ffxtbr.manifest  .
- If the problem is in Thunderbird 2, check for the hidden Offline extension. In Thunderbird's installation directory, go to the "chrome" folder. If any of the files "installed-chrome.txt", "app-chrome.manifest" and "xpinstall.manifest" are there, edit them. Remove any lines that mention messenger-offline (which probably means the files will be empty and you can delete them). Also remove "offline.jar". Other hidden extensions might require similar treatment.
Important: To restart your Mozilla application you must completely close it, including all its windows and those belonging to any extensions (eg. Chatzilla). The easiest way to do this is to use "File -> Exit/Quit".
Related bug reports