Multi-profile support has been in FF since I started using it (at least 5 years ago?). The profile picker isn't as pretty as the Chrome window you get but its definitely there.
My biggest gripe with multi-profile support is that it mixes the taskbar icons in windows. There's a multi-step workaround that involves the registry and cloning programs files folders to fix this specific issue but you need to do it for each new profile and it breaks constantly with updates and doesn't work consistently even in between updates. Yes, containers solve some of the problems that chrome profiles attempt to solve but I still have a work profile and personal profile which I like to keep separate.
Unless there's a way to create containers that also disable extensions in their context I will always need multiple profiles.
> My biggest gripe with multi-profile support is that it mixes the taskbar icons in windows.
edit: never mind, I wasn't awake, see reply.
FWIW on Windows this works fine for me. I run two different profiles. I get two separate taskbar icon groupings, they stay together and collapses independently.
Must admit I've not used multiple profiles on my KDE install yet.
I prefer to NOT group windows so I can sort them manually, but also, offhand, because I believe it groups off of the executable name, not the profile.
If you right click on the Icons-and-Text Task Manager Settings, the Behavior tab has TWO options for Group: behaviors [Do Not Group] (I prefer this anyway) or [By Program Name] which I take to mean the executable. I'm not sure if it follows symlinks, but if that's an issue you can probably make a script to hardlink instead. A couple firefox-profilename applications that point back to firefox should work. You can even make a custom version of the launcher that invokes with a specific profile ID rather than selecting one.
As a further set of asides:
Firefox please 1) It'd be nice if firefox renamed it's arguments list to prefix it with the selected profile ID as the first item.
KDE Plasma please 2) It would be great if the grouping thing could be expanded to E.G. consume the program name + first two tokens of the arguments. Maybe with a customization per group.
KDE mega _please_ 2.b) Or even just to call an external program with a window ID handle and have it write back a string that's used as the group 'name'.
Actually I realized I totally forgot a crucial detail. I run my other profile using Firefox Beta, so they really are separate applications. I assume that would fully explain why they behave separately. Sorry about that.
Will have to test using multiple profiles using the same executable.
However, if the same "trick" works on Linux, perhaps it could be exploited by Firefox creating a separate executable that launches a specific profile.
We do that with our application, using the "self-extracting zip trick", ie appending data to the executable that is read during start-up.
Firefox couldn't do the link thing itself without either being a setuid binary, or running somewhere the user could replace Firefox. You could MAYBE get it to make a symlink under XDG_RUNTIME_DIR, or /tmp if that's unset, since symlinks work across devices. At least on my desktop /tmp is allowed to contain executables and a symlink to /usr/bin/bash executes and remains named as the symlink path.
Though it'd still be many times better if Firefox made it easier to tell which profile was associated with a given process (prefix the argument list as if launched with that). ALSO if KDE's task list supported improved flexibility for group categorization. It wouldn't even need to run an external program other than once for each new window created.
I've also been using it for years, and it seems to work exactly how it should, but it does seem to be a bit of a "hidden feature" in my opinion. It wasn't super easy to figure out that the feature existed and how to use it.
The problem is discoverability and UX. I believe, in chrome profile management is another menu option. In FF you have to use about:profiles or start Firefox with a specific command line argument.
Unless you know about it or are actively looking for the feature, there's no way to know. If you are coming from Chrome, this unfortunately leaves the impression that Firefox doesn't support profiles (a must have feature for some).
It doesn't even hint or show up in settings search. It is as if they don't want anyone to use/know about profiles.
Unless there's a way to create containers that also disable extensions in their context I will always need multiple profiles.