From MozillaZine Knowledge Base

To reset the user-agent string to its compiled-in default for your Mozilla application:

1. Type about:config in the Location Bar, then hit Enter. (For Thunderbird, click "Tools -> Options -> Advanced -> General -> Config Editor" instead.)
2. Enter general.useragent in the Filter box.
3. If any of the preferences still displayed are shown as user set, right-click (context click) the preference and select "Reset".

You can also reset your user-agent string by editing the prefs.js file in the profile folder with your Mozilla application closed. (This may be necessary if the browser crashes at startup loading the Java plugin because of a spoofed user agent - see bug 83376.) Save a backup copy of prefs.js as a precaution, then open the prefs.js file in a text editor such as Notepad, delete any lines beginning with user_pref("general.useragent. and then save the changes.

Warning: If you have a user.js file, make sure that any general.useragent. lines are removed from that file as well, since any preferences set by the user.js file will be set again the next time you restart the application.