@ shantanukendre8821 :

Please ignore   yakesiv181 's   post   -   it's a scam !

Hi shantanukendre8821, please ignore the fake support email address someone is spamming here. A genuine community support volunteer will reply in your thread on this site (https://support.mozilla.org/questions/1289067)

Hi shantanukendre8821, you may have malware that runs at system startup, or which created "Scheduled Tasks" to launch URLs in your default browser at system startup. Our support article suggests well-regarded tools for cleaning up: Troubleshoot Firefox issues caused by malware.

I also suggest turning off the feature that integrates Firefox with Windows 10 startup, so you can rule that out as a potential issue. See: Firefox starts automatically when I restart Windows.

^ Note that the OP is on Windows 7.

You can also try to set another browser as the default browser to see if this is about opening pages in the current default browser.