If you don't have any crash reports, this is going to require some trial and error...

To test how Firefox runs "uncustomized" on your system, could you try:

Create a new Firefox profile

A new profile will have your system-installed plugins (e.g., Flash) and extensions (e.g., security suite toolbars), but no themes, other extensions, or other customizations. It also should have completely fresh settings databases and a fresh cache folder.

With Firefox closed, start up in the Profile Manager using Start > Run:

firefox.exe -P

Any time you want to switch profiles, exit Firefox and return to this dialog.

Click the Create Profile button and choose a name like TEST. Please skip the option to change the folder location. Then start Firefox in the new TEST profile.

Does Firefox start up and run any better?

Before exiting, could you copy your settings information from the Troubleshooting Information page. Either

• "3-bar" menu button > "?" button > Troubleshooting Information
• Help menu > Troubleshooting Information
• type or paste about:support on the address bar and press Enter

In the upper left corner, click "Copy text to clipboard" to copy the page contents, then you can paste them into Notepad, Word, or some other temporary place, or directly into a reply here.

When returning to the Profile Manager, you might be tempted to use the Delete Profile button on your TEST profile. But... it's a bit too easy to accidentally delete your "real" profile, so I recommend resisting the temptation. If you do want to clean up later, I suggest making a backup of all your profiles first in case something were to go wrong.

By the way, when you reinstall Firefox, some stray files in Firefox's program folder may remain. You could re-run the installer to create a clean program folder. No uninstall is required, just hide the existing program folder and re-run the installer.

(1) If needed, download a fresh installer from https://www.mozilla.org/firefox/all/ to a convenient location. (Scroll down to your preferred language.)

(2) Exit out of Firefox.

(3) Rename the folder

C:\Program Files\Mozilla Firefox

to

C:\Program Files\OldFirefox

(4) Run the installer you downloaded in #1. It should automatically connect to your existing settings.

Any improvement in startup or stability?

Note: Some plugins may exist only in that OldFirefox folder. If something essential is missing, look in these folders:

• C:\Program Files\OldFirefox\Plugins
• C:\Program Files\OldFirefox\browser\plugins

Also, in case this is the work of malware, please try several of the tools in our support article: Troubleshoot Firefox issues caused by malware.

You can also do a malware check with several malware scanning programs on the Windows computer. Please scan with all programs because each program detects different malware. All these programs have free versions.

Make sure that you update each program to get the latest version of their databases before doing a scan.

• Malwarebytes' Anti-Malware:
  http://www.malwarebytes.org/mbam.php
• AdwCleaner:
  http://www.bleepingcomputer.com/download/adwcleaner/
  http://www.softpedia.com/get/Antivirus/Removal-Tools/AdwCleaner.shtml
• SuperAntispyware:
  http://www.superantispyware.com/
• Microsoft Safety Scanner:
  http://www.microsoft.com/security/scanner/en-us/default.aspx
• Windows Defender:
  http://windows.microsoft.com/en-us/windows/using-defender
• Spybot Search & Destroy:
  http://www.safer-networking.org/en/index.html
• Kasperky Free Security Scan:
  http://www.kaspersky.com/security-scan

You can also do a check for a rootkit infection with TDSSKiller.

• Anti-rootkit utility TDSSKiller:
  http://support.kaspersky.com/5350?el=88446

See also:

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