I use firefox with proxy setting. I use squid proxy server which is installed locally on my system.
I observed that many time clicking on firefox button doesn't work. if you click double it says that , firefox is already running. i was lucky that i found that this is due to squid proxy. Stopping the squid service will help you. after correct startup of firefox again start it.
Now if squid server is on remote location that you cannot edit the setting.
i have not checked answer for this question, may be you need to open the firefox in command line as off-line working mode, or detach the network cable, or may be you need to go firefox directory and edit the configuration. (I have not checked out any of these method , as i use local proxy server).