It is possible that security software (anti-virus, firewall) is causing the problem. Try to disable security software temporarily to see if that makes a difference.

Boot the computer in Windows Safe Mode with network support (press F8 on the boot screen or hold down the Shift key) to see if that has effect.

• http://www.bleepingcomputer.com/tutorials/how-to-start-windows-in-safe-mode/
• http://www.7tutorials.com/4-ways-boot-safe-mode-windows-10

I turned off my security software (Bitdefender) and thus solved the problem but this is an unwise scenario. The primary reason for having anti.virus is especially needed when surfing the web. Is this a Mozilla defect or what?

You posted here with Firefox 45.0, why the need for downloading Firefox again? Or are you wanting to download the 32-bit version of Firefox? If not, can't you use Help > About Firefox to manually trigger an update to Firefox 46.0?

As far as a "defect" goes, if anything that would be BitDefender not realizing that Firefox 46.0 was released on Tuesday 04-26 (today). Sometimes these AV / Firewall applications lag a day or two after Mozilla releases an new version to update their signature files or they wait until they get a lot of user complaints before fixing an issue such as this.