Hello Syed.

A screenshot of the error message would be helpful.

In any case, you may be having a problem with some extension or plugin that is hindering your Firefox's normal behavior. Have you tried disabling all add-ons (just to check), to see if Firefox goes back to normal?

See also http://kb.mozillazine.org/Firefox_crashes and Troubleshoot Firefox crashes (closing or quitting unexpectedly)

Whenever i tried to enter into gmail it closes showing an error message

Yes, that's probably some add-on you have in your Firefox. Run Firefox in safe mode to temporarily disable all add-ons (extensions, themes and plugins) and see if the problem persists.