An external application sending a URL to Firefox only works with the Default Browser in Windows set as Firefox, which will then use the Default =1 Profile as listed in the profiles.ini file. Trying to open the Profile Manager and then selecting a Profile doesn't work with an external application. It works best (/only?) by going thru the Profile Manager feature, without using the PM interface. If the Default profile is already running, a new window should open with the "sent" URL. And if you have another Profile running, you're not going to get the Default to open; it will tell you "Firefox is already running". It ''is'' possible to open a Profile when another is already running, by using the '''-no -remote''' command, but that command line parameter won't work with the Default browser / Default Profile combo. I might have a "solution" if I knew why you are using multiple Profiles. If are interested in learning "how" to deal with running multiple Profiles, see this: http://forums.mozillazine.org/viewtopic.php?f=23&t=2821799 And at the bottom of that article are links to 4 previous version of that article. Keep in mind that all 5 of those articles were written from a different perspective than what you are asking about; multiple different Firefox installations, with multiple different Profiles, to allow a person to use Firefox Release smoothly, while "testing" the pre-release versions that will become a Firefox Release in the near future. You'll need to "pick out" what applies to what you want to do when reading those postings, and forget about the multiple different Firefox versions. The "drill" as it pertains to Default / Default is basically the same, regardless of how many "Firefox" program installations you have.