First, thank you for you donations. Now, click settings>general and untick the setting to show startup page.

Thanks for the answer. But.... I did it several times. Any change I make in the settings are NOT BEIING saved. Every time I start TB again... all is back like it was before... That's the problem...

This hints of a problem in saving file to hard drive. Thunderbird saves settings in prefs.js, a file in the profile. It does that when closing and, there is an error in saving, when thunderbird restarts, it uses the prior setup. You might check in profile to see if that file is read-only. Problems can also happen if a VPN or antivirus is interfering with thunderbird. You might see multiple copies of prefs. js, such as prefs-1.js, prefs-2.js, etc. In that case, the one with highest number 'may' be the one with latest settings. If the update date reflects that, you could delete all prefs.js files except the highest one, and rename it to prefs.js. These are simple text files, so you could also browse them for content.

Thanks 1000times David!! My problem is solved!!!

I am so glad to have helped. You are most welcome. :)