What do people use for “Profiles”. I need ability to switch easily between profiles (different identities) without going to command line. I also don’t want to install a third party extension for that if possible
Firefox has a Container Tabs extension which does just this - the defaults are profiles for Work, Personal, Banking, and Social, and you can add more. You do have to install it but I think the extension is from Mozilla itself, so it's not exactly third-party. It's just features that are already in the nightly builds as I understand it [0]. Personally I've really enjoyed it so far!
The extension also allows you to set that a particular site be assigned to, and automatically open in, a given container. I don't believe this particular feature is built into Firefox.
Indeed, it's just the containerisation functionality and assigning a tab to open in a Container that's built in; actually initiating the opening is done by the extension, e.g. when you navigate to a specific origin.
On macOS, instead of running Firefox via the app icon, I run a an AppleScript[0] that forces Firefox to launch with the Profile Manager window. I have it bundled as a one-step Automator application, which I run from Spotlight but which can easily be added to the Dock or whatever.
I used to have this script built as an app bundle via Platypus, but that stopped working somewhere between Yosemite and Mojave (likely due to increasingly strict OS file permissions or something). The Automator app doesn't seem to suffer from this issue...I think I just had to grant it a couple permissions the first time I ran it.
I wasn't able to figure out how to set a custom icon from within Automator, but it's not too hard to do if you're comfortable mucking around inside app bundles. The icon file is stored under <app-bundle-root>/Contents/Resources/, and its name (sans extension) is referenced in <app-bundle-root>/Info.plist.
