Did you try this?

http://kb.mozillazine.org/Browser_will_not_start_up#No_Mozilla_applications_will_start

This can be caused by an incorrect setting in the Windows registry. If you attempt to install the Opera browser as a workaround, or if you attempt to install or remove various Microsoft applications, you will see an error similar to, Error 1606: Could Not Access Network Location. See this Microsoft support article for details, including a fix you can download.

The reference for the above is http://forums.mozillazine.org/viewtopic.php?f=38&t=1986611

I have already tried the downloadable fix and it did not work. I will try to look manually into the registry later, as I cannot dedicate time to it now.

Thanks anyways !

Until you figure it out you could try "Portable Firefox" (which was mentioned as working in the mozillaZine thread I linked earlier):

• http://portableapps.com/apps/internet/firefox_portable

Support for this third-party application is here, if you need it: http://portableapps.com/support/firefox_portable

I finally had time to look into the registry, and everything was fine. I also tried "Portable Firefox", and it did not work, exactly like Firefox and Thunderbird did.

I am quite lost here..

It is possible that your security software (firewall, anti-virus) 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

• https://support.mozilla.org/kb/fix-problems-connecting-websites-after-updating

Boot the computer in Windows Safe mode with network support (press F8 on the boot screen) as a test to see if that helps.

I put both my firewall and antivirus on "off", granted Firefox it's access again, and nothing worked.

I had already checked under Windows Safe mode and Firefox was already not working.

Still, thanks for trying !

Does the portable Firefox version work?

• http://portableapps.com/apps/internet/firefox_portable

Do a malware check with some malware scanning programs on the Windows computer.
Please 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.

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

The portable version does not work.

I will be doing the scans over the course of the night, and will report back later. Thanks !

Reporting back, I tested my computer with all the programs you listed, cleaned some things with 'em (mostly cookies..) but things are still not working.

Worth mentionning : I am having the same trouble when I launch The Secret World and Borderlands 2 . I am considering a full reset of the computer, or throwing it out the window by now.