The related bug for that crash report is 797193 NEW --- Firefox startup crash in nsFileStreamBase::Read

I don't see anything that useful in the bug report so you should start here: Troubleshoot and diagnose Firefox problems

See if you can start in Firefox Safe Mode (hold down the SHIFT key while you doubleclick the Firefox icon, then click the "Start in Safe Mode" button). If that works, see: Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems

• Link: https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

P.S. If Internet Explorer is also crashing at startup and if Firefox still crashes even in Firefox Safe Mode, then your problem may be a result of malware installed on your computer.

You can try these free programs to scan for malware, which work with your existing antivirus software:

• Microsoft Safety Scanner
• MalwareBytes' Anti-Malware
• TDSSKiller - AntiRootkit Utility

Microsoft Security Essentials is a good permanent antivirus for Windows 7/Vista/XP if you don't already have one.

Further information can be found in the Troubleshoot Firefox issues caused by malware article.

This looks like a crash related to reading a pref file.

You can check for problems with preferences and try to rename or delete the prefs.js file and possible numbered prefs-##.js files and a possible user.js file to reset all prefs to the default values.

• http://kb.mozillazine.org/Preferences_not_saved
• http://kb.mozillazine.org/Resetting_preferences

Create a new profile as a test to check if your current profile is causing the problems.

See "Creating a profile":

• https://support.mozilla.org/kb/profile-manager-create-and-remove-firefox-profiles
• http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Profile_issues

If the new profile works then you can transfer some files from an existing profile to the new profile, but be careful not to copy corrupted files.

• http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox