You have to add the -no-remote parameter to your shortcut.

madscientist2 said

On our home computer we have multiple firefox profiles. Since updating to Firefox 57 we can only use one at once. Opening firefox opens the default profile. About:profiles lists all the profiles, but clicking the "Launch profile in new window" simply opens another window of the current profile, it does not open the new profile.

That button doesn't work if you are currently running Firefox in the default profile. As noted in the first reply, you may need to add -no-remote to your shortcuts.

https://developer.mozilla.org/docs/Mozilla/Command_Line_Options#-no-remote

Thank you TyDraniu, that made the shortcut work. jscher2000, why does opening a new profile not work for the default profile. To me this is a severe limitation and appears to make it that much harder to use profiles. If I understand correctly to actually use an alternative profile you have to create a shortcut to another profile using the correct parameters. About:profiles is useless for opening other profiles unless you have a shortcut to first open an alternative profile. What is the rationale behind this limitation?

madscientist2 said

What is the rationale behind this limitation?

IMO, that is an incomplete or unfinished feature. No rationale, just releasing a new feature before it is ready for release. Aw, we'll fix it later. It works good enough as it is to be released. But how many times does Mozilla not actually complete the new feature? I quit counting a few years ago.

From my perspective as a non-programmer, but fairly advanced level user, I have "seen" years of internal battles (in Bugzilla) over providing user access via a user interface. One "major player" has been against it for a long time, even "threatening" the removal of "Profiles" altogether repeatedly in various Bug reports where a user requested changes to make it easier and have a better user facing interface. "Not necessary for the average user" was the typical retort."

Then sometime well after Firefox 4.0 was released, it was decided to offer a "Profiles UI" for the alpha Nightly builds, and later the "new" (at that time) Developer Edition alpha2 builds. That started as "toggling" between a "tester" Profile and a "release version" Profile to allow "testers" to test on the Nightly or DE versions, while still having a Profile to use with the Release or Beta versions of Firefox; thus avoiding potential problems between Alpha testing and using a Release version. Meanwhile just before that, some summer interns created an XUL-Runner external Profile Manager that "worked a treat", which was never completed but did work quite well. But, a lot of the code that they wrote for that external Profile Manager made it into Nightly, and the DE builds Profile Manager scheme. And a few years later Mozilla decided to kill off XUL-Runner which "killed" the external application PM (and more currently, kill XUL in Firefox altogether).

The Nightly / DE feature was never 100% complete (IMO) but the users of those builds were able to cope with the deficiencies. When about:profiles made it into the Release versions I don't think enough priority was put on finally finishing the job.

Personally I have been messing with running multiple versions of Firefox with multiple Profiles for like 14 years now, and have written a number of support thread over at the original Mozilla support fora - long before Mozilla started this support fora in 2007 - 2008.

Version 5 of the threads I started on the subject of "Profiles" over at the mozillaZine fora. http://forums.mozillazine.org/viewtopic.php?f=23&t=2821799 And at the bottom are links to the 4 previous versions of that support thread. Can be very confusing to read, as I was trying to cover a variety of different reasons for a person to need / use multiple Firefox Profiles and multiple different Firefox version being installed. And I included all the "tricks" that other users provided over the years in the first two versions of that topic, when those threads weren't "locked" to additional postings.

I became involved deeply with "messing with Profiles" after changes that Mozilla made to "Profiles" and a new Profile Manager that came about with Firefox 0.9 in early-2004. The method that I was using in Firefox 0.8 quit working and with the new for Fx 0.9 Profile Manager I had problems with the older 0.8 version sometimes opening unexpectedly - which drove me crazy, until I built a new PC and didn't install all the older versions of Firefox. (Yeah, I am a hoarder of software, mostly Firefox versions. I rarely uninstall the old versions when installing the latest version. Easy for me to return to the older version when I am learning the "tricks" on the new version.)

So, Firefox 57 hasn't interfered with my use of Firefox at all. I "play" with Quantum to see how I can configure it to my needs and pattern of usage, while being able to continue using an older version (or 3). My timeline for using Quantum most of the time runs to Firefox 60 or Fx 61