Is it happening to particular website or all websites?

It is happening on all websites.

I am experiencing this behaviour since the last update to 24.5.0 It has worked perfectly before. One interesting thing. Downgrade does not solve the issue!

To diagnose problems with Thunderbird, try one of the following:

• Restart Thunderbird with add-ons disabled (Thunderbird Safe Mode). On the Help menu, click on "Restart with Add-ons Disabled". If Thunderbird works like normal, there is an Add-on or Theme interfering with normal operations. You will need to re-enable add-ons one at a time until you locate the offender.
• Restart the operating system in safe mode with Networking. This loads only the very basics needed to start your computer while enabling an Internet connection. Click on your operating system for instructions on how to start in safe mode: Windows 8, Windows 7, Windows Vista, Windows XP, OSX

If safe mode for the operating system fixes the issue, there's other software in your computer that's causing problems. Possibilities include but not limited to: AV scanning, virus/malware, background downloads such as program updates.

Unfortunatelly, both options did not solve the issue. The behaviour is the same regardless of tested option.

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

• http://www-archive.mozilla.org/support/thunderbird/profile

If the new profile works then you can transfer files from a previously used profile to the new profile, but be cautious not to copy corrupted files to avoid carrying over the problem

• http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Thunderbird
• http://kb.mozillazine.org/Profile_Manager