Last week my old dev system bit the dust. Since the box was 6 years old, I figured I was better off replacing the unit rather than repairing it—especially considering it was the motherboard that got fried (which would have meant replacing the motherboard, CPU, video card and possible power supply since all components were older technology.)
Anyway, in the process of setting up my system, I finally thought it was time to change from using Firefox 2 as my default browser and use Firefox 3 instead. I've been running both browser simultaneously for a while, but had just kept using FF2 mainly for development purposes.
When I initially set up the new box, I used the same --no-remote technique I had previously blogged about so that I can have both Firefox 2 and Firefox 3 open at the same time. However, whenever I'd try to open up a link from Outlook when Firefox 3 was already running I'd see the following error:
After doing some Googling and not finding anything that matched my issue, it finally occurred to me that the problem might be stemming from the fact that I was initially opening my Firefox 3 instances with the --no-remote switch still.
Sure enough, after removing the --no-remote switch from my quick launch icon, the error went away. I know only start Firefox 2 with the --no-remote switch.
So, if you're seeing the "Firefox is already running, but is not responding." message when you try opening up links from applications (such as your e-mail client) make sure you're not opening your instance of Firefox initially with the --no-remote switch.