[pleb_nz]

Firefox does have profiles and has had for as long as I’ve needed them. For some reason they’ve always been hidden away and needed different args on start to change the profile being started, or the use of an extension to allow profile switching. I don’t use them anymore but from what I remember they worked well and were very isolated. Hopefully they’re using the current implementation and just making it more user friendly?

[worble]

You can use the url about:profiles to manage profiles too, which is still hidden away and not a great UI, but at least you don't need to use an extension or the command line.

[akdor1154]

As of a recent update, the Profile Manager is available as a startup option if you right click the ff icon (on gnome at least, i assume this was implemented cross platform)

[yjftsjthsd-h]

It's not just a GNOME thing, but it might not be that cross-platform. On Linux, there's a firefox.desktop file that describes the application to your DE/launcher, and it looks like this on my box:

    [Desktop Entry]
    Actions=new-private-window;new-window;profile-manager-window
    Categories=Network;WebBrowser
    Exec=firefox --name firefox %U
    
    ---snip---
    
    [Desktop Action profile-manager-window]
    Exec=firefox --ProfileManager
    Name=Profile Manager

So that'll work on anything that uses https://specifications.freedesktop.org/desktop-entry-spec/de... and there could be parallel features for Windows/Mac, but there also might not be or Firefox might not implement them.

[lxgr]

It's definitely not an option on macOS, at least.

Holding the "option" key is even somewhat of an UI convention on macOS to allow opening an alternative media library (e.g. for Photos, Music aka iTunes etc.), yet Firefox maps it to "startup in safe mode".