Please follow the troubleshooting information in firefox crashes use only the default theme and disable all plugins.

If that does not help try a new profile, and also try Windows SafeMode (usually accessible by using the f8 key on startup))

• to create a new profile see basic troubleshooting_8-make-a-new-profile

Unfortunately the CrashID was not much use
( maybe you will have others after trying a clean install )

• CrashID 05a3e4e1-fb00-42db-acfe-9bb932120325
  • Signature: (null signature)
  • (Firefox 12.0 Crash Report [@ EMPTY: no crashing thread identified; corrupt dump ] )

You can also do a malware check with some malware scanning programs on the Windows computer.
You need to scan with all programs because each program detects different malware.
Make sure that you update each program to get the latest version of their databases before doing a scan.

• http://www.malwarebytes.org/mbam.php - Malwarebytes' Anti-Malware
• http://www.superantispyware.com/ - SuperAntispyware
• http://www.microsoft.com/security/scanner/en-us/default.aspx - Microsoft Safety Scanner
• http://www.microsoft.com/windows/products/winfamily/defender/default.mspx - Windows Defender: Home Page
• http://www.safer-networking.org/en/index.html - Spybot Search & Destroy
• http://www.lavasoft.com/products/ad_aware_free.php - Ad-Aware Free

See also:

• "Spyware on Windows": http://kb.mozillazine.org/Popups_not_blocked