Firefox instances using profiles are grouped in a single group

problem: Windows from separate Firefox instances using profiles are grouped in a single group on the taskbar 

Lets say you have 3 Firefox profiles (about:profiles), or 3 portable Firefox instances, and you have the separate instances all running at the same time. I had the issue that all the windows from the various profile instances were grouped into a single group on the task bar, and I could not find a way to un-group them.

Until recently I've been primarily been a Chrome user, so I was used to separate profile instances having their own taskbar group. Excessive CPU/resource utilisation aka fan noise and less battery have pushed me back towards Firefox.

impact: hard to know which windows relate to which profile

This problem impacts productivity and increases frustration because you've no immediate idea which windows relate to which profiles. If you're like me and like to keep certain topics clean and separate, like work and private life, and various project work, this is a major issue.

solution: there is a config tweak workaround

There was an official bug fix for this issue titled "No way to un-group separate instances on Windows 7 TaskBar". It took me at least an hour of research to find and get this setting working after a number of dead ends, so sharing it here.

1. Open about:config
2. change browser.startup.blankWindow and the value to false.
3. Right click "new" > "boolean"
4. When prompted set the name to taskbar.grouping.useprofile and the value to true.
5. Restart Firefox

Note that the new setting does not use the browser. prefix.

citation:

Props to:
ptsampoukas @ MozillaZine forums for pointing me in the right direction.