Detect the type of user agent (IE?FF?)

PHP:
http://www.diybl.com/course/4_webprogram/php/phpjs/2008515/116115.html
ASP:
http://www.zychina.com.cn/article/160/184/4511.html
JAVASCRIPT:
http://www.cnblogs.com/leadzen/archive/2008/09/06/1285764.html
WIKI:
http://en.wikipedia.org/wiki/User_agent
Others:
http://blog.csdn.net/perddy/archive/2008/04/11/2282287.aspx

ffx addon:
https://addons.mozilla.org/en-US/firefox/addon/967

File for about:config in Firefox

I want to backup my about:config settings, where is the file that stores it?

I think you mean this file:

prefs.js

Look for it in

/home/your_user/.mozilla/firefox/orws2gsm.default

Settings for crash restore: 

user_pref("browser.sessionstore.resume_from_crash", false);