Start Firefox in Use Troubleshoot Mode in Firefox to check if one of the extensions is causing the problem (switch to the DEFAULT theme: Firefox (Tools) > Add-ons > Appearance/Themes).

• Don't make any changes on the Safe mode start window.
• https://support.mozilla.com/kb/Safe+Mode

• https://support.mozilla.com/kb/Troubleshooting+extensions+and+themes

Thanks cor-el but this doesn't fix the problem. Additional info: Now if I close Firefox and reopen, without rebooting the computer the proxy is again not saved. I have done another scan for virus/malware but machine appears clean. Internet Explorer is working fine and has saved proxy settings.

It appears to be a Plugin causing the problem. Getplusplusadobe16244. I systematically ran through every add on/plug in and disabling the above stops the problem happening. Tks Cor-el, gave me the reason to check everything.

Did you try to delete the file prefs.js and did a check for the presence of a file user.js in the Firefox Profile Folder?

• http://kb.mozillazine.org/Profile_folder_-_Firefox
• Help > Troubleshooting Information > Profile Directory: Open Containing Folder

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

See Basic Troubleshooting: Make a new profile:

• https://support.mozilla.com/kb/Basic+Troubleshooting#w_8-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" in case there are still problems.

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