Firefox error "firefox is already running" ALMOST fixed...

[zombifier25]

killall firefox

To get rid of all Firefox processes, if any. And then start Firefox is Safe Mode:

firefox -safe-mode

and see if any addons is causing the malfunctioning.

[Marylouise]

I have been having this same issue. A few minutes ago I closed the Firefox is already running window. I then the tried to open Browser from a different location. Alls well!! The Firefox already running did not open.

For some unknown reason if I use the icons on the desk top it does it. And if I just the tab on top left above the desktop platform, it works just fine. Can anyone explain this to me?

[rich1842]

Sounds as if the desktop shortcut is broken. I would delete it and make a new one (go through the menus to applications - internet - rightclick on firefox and choose 'add to desktop').