Hi, You can try starting Firefox in a [https://support.mozilla.org/en-US/kb/Managing-profiles new profile] and later [https://support.mozilla.org/en-US/kb/Recovering%20important%20data%20from%20an%20old%20profile?s=profile&r=1&e=sph&as=s copy] the personal settings/data from the previous profile. [https://support.mozilla.org/en-US/kb/Profiles?s=profile&r=2&e=sph&as=s Profiles Howto] [https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles Profile Manager] [http://kb.mozillazine.org/Profile_folder_-_Firefox Profile Folder & Files] Also, If you have security software (antivirus, firewall etc.) installed, you can try deleting all existing instances of Firefox and its related processes/files in all the different configuration modules/areas of the security software like process monitoring, sub/spawned process control, virtualization, HIPS etc., in addition to the security software's main configuration panel. Instead create new fully allowed/trusted rules for Firefox + its related processes in all the different modules/panels - a genuine/original Firefox installer as well as all the installed EXEs (Application) and DLLs (Application extension) are digitally signed by Mozilla Corporation (right-click > Properties > Digital Signatures). Even otherwise, some security software may also ask again when Firefox and/or its sub processes are started, and you may be able to allow/trust at that time. Please see [https://support.mozilla.org/en-US/kb/Firewalls this]. [http://kb.mozillazine.org/Firewalls AV/Firewalls Configuration]

Hi dumdidadida Many thanks for your quick reply. I followed your comments about anti-virus but there were no problems and conflicts between my kaspersky and mozilla-firefox. By the way, that was fruitful for me for other similar problems I may have in future :) Finally I had to download the latest mozilla and reinstall my firefox. Fortunately it just upgraded my firefox and now it works again. Be happy

Thanks for the detailed update :)