Hi, go to about:preferences and check the option that says 'Allow Developer Edition to Run Alongside Firefox' -- This will stop this from happening again.

Hi Mkll, "Allow Firefox Developer Edition and Firefox to run at the same time" is already checked.

Make sure you use dev-edition-default as the profile for Developer Edition!

I have tried to reproduce this on other machine and I think this happens because I am launching firefox with -no-remote flag. So, the exact step to reproduce this behavior: 1. Create two profiles named A and B. 2. Launch firefox with -no-remote flag and open firefox with profile A (might need to use -P flag to see profile select window). 3. Click on a link somewhere outside the browser (pdf file, outlook and so on). This should open profile select window again. 4. Select profile A and click on start firefox. This should throw "Firefox Developer Edition cannot use the profile “profileA” because it is in use. To continue, close the running instance of Firefox Developer Edition or choose a different profile."

Again, the result that I expect: after choosing the profile, link should be opened in the new tab of selected profile (in this case in profileA).

I guess the issue is the -no-remote flag and removing it would fix the issue. I am using this flag so that I would be able to run multiple firefox instances at the same time and that each instance could use a different profile. I am not sure if this is the best way to do that but it is the only one I found. I need to be able to do that, so I can't just remove -no-remote flag, but not being able to open links with a running profile is really annoying.

Any ideas?

You can't open external links in a Firefox instance started with the -no-remote switch. You have to omit this switch from the profile you want to open external links in and only use -no-remote to open extra profiles. That is how -no-remote works: it makes the Firefox instance invisible. You can drag a link to such an instance (e.g. to the new tab + button) as a workaround.