Do a clean reinstall and delete the Firefox program folder before (re)installing a fresh copy of the current Firefox release.

Download a fresh Firefox copy and save the file to the desktop.

• Firefox 27: http://www.mozilla.org/en-US/firefox/all.html

If possible uninstall your current Firefox version to cleanup the Windows registry and settings in security software.

• Do NOT remove personal data when you uninstall your current Firefox version, because all profile folders will be removed and you lose personal data like bookmarks and passwords from profiles of other Firefox versions.

Remove the Firefox program folder before installing that newly downloaded copy of the Firefox installer.

• (32 bit Windows) "C:\Program Files\Mozilla Firefox\"
• (64 bit Windows) "C:\Program Files (x86)\Mozilla Firefox\"

• 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.
• http://kb.mozillazine.org/Uninstalling_Firefox

Your bookmarks and other personal data are stored in the Firefox profile folder and won't be affected by an uninstall and (re)install, but make sure that "remove personal data" is NOT selected when you uninstall Firefox.

If you keep having problems then also create a new profile.

• http://kb.mozillazine.org/Profile_folder_-_Firefox
• http://kb.mozillazine.org/Profile_backup

• http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Clean_reinstall

Sir
thank you for your reply
before posting my question i have uninstalled version 26 several times and completely delete firefox folder.
After installing version 27 will not start and display this in a small window :
XML Parsing Error: undefined entity
Location: chrome://mozapps/content/extensions/update.xul
Line Number 14, Column 1:<wizard id="updateWizard"<br /> ^
Cannot even start in safe mode
I have Lenovo G550 laptop running win XP SP3
I have also try v 27.01 but same problem
please advice

I couldn't open v27 and used 26, hoping that the next version will solve the problem . 27 just didn't open, with no fault message. The message mentioned above appeared with v28. So I'm back with v26...