It won't.

That being said, Windows XP itself will be insecure. You can use an up-to-date browser, plug-ins, firewall, anti-virus and anti-malware software. But that won't change the fact the underlying foundation will contain security vulnerabilities that will never be patched and attackers will be aware of them.

• https://support.mozilla.org/questions/989084
• http://windows.microsoft.com/en-US/windows/end-support-help

hello, mozilla will also continue to support windows xp after april and continue providing feature and security updates for firefox for the time being.
this does of course not guarantee that you can continue to browse safely - if your OS gets infected through any exploitable security vulnerabilities other-how, this can/will also compromise all browsers on your system and your personal data (like passwords, history, and so on) they contain.

Yes.
Windows XP with service pack 2 or 3 is still a supported configuration, so as far as regarding Firefox you will still get security updates.
You will have to make sure that your security software will be updated as well and keeps supporting Windows XP.

• http://www.mozilla.org/en-US/firefox/28.0/system-requirements/

What can I do in order to be sure my windowsxp is completely protected?

jeanniebeach wrote:

What can I do in order to be sure my windowsxp is completely protected?

1. Keep Firefox updated.
   • Update Firefox to the latest release
2. Keep your plug-ins updated.
   • https://www.mozilla.org/plugincheck/
3. Use a good firewall and keep it updated.
   • Outpost • Comodo • Comparison of firewalls
4. Use a good anti-virus and keep it updated.
   • Avast • Comparison of anti-virus software
5. Use good anti-malware software and keep it updated.
   • Malwarebytes Anti-malware • Malware protection software

All of the above will be like installing a top-of-the-line security system in your home, while having a giant, gaping hole in the side of the wall that burglars can easily walk through.