[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".