why is my macbook rebooting all the time especially when using Safari, Chrome or Firefox ?

No idea, however you can start a process of elimination procedure to fix everything on your Mac to narrow down where the problem is.

Step by Step to fix your Mac

I have no idea - note the exact time the next time it happens and then open Console and copy and paste the crash log here.

Also post which model MBP you have and the version of OS X that you're running.

Clinton