Hi parab, sorry to hear about the crashing.

Will Firefox run for a few minutes or does it crash immediately when you try to start it?

When it crashes, Firefox usually records some data about what it was doing at the time. If you submit your crash data to Mozilla, you can post a code here that allows volunteers to look at that data. This article (last section) describes how to find those crash IDs: Troubleshoot Firefox crashes (closing or quitting unexpectedly).

I reinstall the firefox many times but it crashes immediately when i tried to start it so plzz help me to fix this problem if u can...!!  :(

One reason that Firefox might crash immediately on startup is an external program named BitGuard that has become more of an issue recently. This article has more information: Uninstall BitGuard to fix crashes with Firefox.

You also might try starting Firefox in "Safe Mode" (Firefox's Safe Mode, not Windows Safe Mode). To try that, hold the Shift key down when starting Firefox.

In the small dialog that (hopefully) appears, click "Start in Safe Mode" (not Reset).

Will Firefox start? More info on Safe Mode: Diagnose Firefox issues using Troubleshoot Mode.

By the way, the crashes article describes how to find crash IDs even if you can't start Firefox, but it is more work...