Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of your add-ons is causing your problem (switch to the DEFAULT theme: Tools > Add-ons > Themes).

See Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems and Troubleshoot issues with plugins like Flash or Java to fix common Firefox problems

If it does work in Safe-mode then disable all your extensions and then try to find which is causing it by enabling one at a time until the problem reappears.

You can use "Disable all add-ons" on the Safe mode start window to disable all extensions.

You have to close and restart Firefox after each change via "File > Exit" (Mac: "Firefox > Quit"; Linux: "File > Quit")

yah ..i tried in safe mode ...its still same as it was ...& im usin the default theme ....dint installed ny plugin or extension xcept ....flash & java ....please do sumthing ...what kind of problem is this ?!!!

Create a new profile as a test to check if your current profile is causing the problems.

See Basic Troubleshooting: Make a new profile

There may be extensions and plugins installed by default in a new profile, so check that in "Tools > Add-ons > Extensions & Plugins"

If that new profile works then you can transfer some files from the old profile to that new profile (be careful not to copy corrupted files)

See http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox

If that doesn't help then do a clean reinstall and and download a fresh Firefox copy from http://www.mozilla.com/firefox/all.html and save the file to the desktop.
Uninstall your current Firefox version and remove the Firefox program folder before installing that copy of the Firefox installer.
Don't remove personal data when uninstalling. It is important to delete the Firefox program folder to remove all the files and make sure that there are no problems with files that were leftover after uninstalling.