This might be an issue related with the drivers for the mouse after installing your OS; you may try updating the drivers for the mouse from the Device Manager and check.

Start Firefox in Safe Mode to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem.

• Switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance
• Do NOT click the Reset button on the Safe Mode start window

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

Is this a new issue following the Windows 10 upgrade?

Firefox 40, scheduled to be released soon, will contain numerous compatibility fixes for Windows 10, so hopefully that will help if there isn't any other explanation or fix for Firefox 39.

Drunquelt said

In text boxes in websites like Codecademy my cursor is not there where i am giving input from keyboard!

Yes! this is an issue after win 10 upgrade...okay i will wait for that!!