First, make sure your version of Windows is all up to date. If you don't have the latest Service Packs for windows XP, Firefox won't start. You may have to run update a few times to get them all.

Then, download and reinstall Firefox from www.getfirefox.com

A possible cause is security software (firewall,anti-virus) that blocks or restricts Firefox or the plugin-container process without informing you, possibly after detecting changes (update) to the Firefox program.

Remove all rules for Firefox and the plugin-container from the permissions list in the firewall and let your firewall ask again for permission to get full unrestricted access to internet for Firefox and the plugin-container process and the updater process.

See:

• https://support.mozilla.org/kb/Server+not+found
• https://support.mozilla.org/kb/Firewalls

See also:

• http://kb.mozillazine.org/Browser_will_not_start_up