HI, I have not seen this before in the info that was sent with your question. Your profile folder contains a user.js file, which includes preferences that were not created by Firefox. Important Locked Preferences

Am not really liking the above and this being installed : IObit Surfing Protection & Ads Removal (is turned off but still is adware as is the whole Advanced System Care) Are you running the whole Advanced System Care package of software by iObit ?

Ok well let's try and fix your browser up. This takes less than 10mins to do. Please : uninstall Firefox. Then Delete the Mozilla Firefox Folders in C:\Program Files and C:\Program Files(x86) Then restart system. Then run Windows Disk Cleanup. (Note: This should be Pinned and run Weekly, If never done below expect 10's of gig's) Then run it again and click the button that says Cleanup System Files. Note: your Firefox Profile is saved. But you should make a back up before you do :

• https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles
• https://support.mozilla.org/en-US/kb/export-firefox-bookmarks-to-backup-or-transfer

Reinstall with Current Release Firefox 58.0.2 with a Full Version Installer

• https://www.mozilla.org/firefox/all/

Please let us know if this solved your issue or if need further assistance.

How are you opening another Firefox instances?

Are you using the -no-remote command line switch in the target line of the Firefox desktop shortcut?

If you use this switch then you can't open an external in a running Firefox process because using this switch hides that Firefox instance.

• https://developer.mozilla.org/Mozilla/Multiple_Firefox_Profiles

I was clicking on an internet shortcut on my desktop and the browser did not start and open the requested page. I later tried to open a second tab on an already open instance of Firefox and that also failed.

The previous suggestion from Pkshadow partially solved the issue but from following his suggestion I also found that Advanced System care had configured itself to prevent all other instances, apart from the first instance, of the browser to open. I have reconfigured ASC and all appears to be OK now. The issue now is WHY did ASC do this to begin with?

Hi, generally speaking ASC is not a reputable company when it comes to running Ad and Malware Cleaners they all want to delete it because it keeps giving ads to you to buy it or download another program of theirs to try/buy.

As to why it mucked with the settings, most of these programs do and you need to go through every setting to make sure it does not delete stuff or change things.

Am glad you got figured out. Please Mark this as Solved with the Solution that worked (yours).