I’m confused; doesn’t your OP and the screenshot show the opposite? On a freshly installed Ubuntu 26.04 (GNOME) Desktop, I can just click the “lock” icon in the address bar to the left of a URL and drag it to the Desktop; or just select the whole URL and drag that.
I’m not sure what you mean by multiple instances; multiple windows are supported, as others have pointed out. If you’re referring to running instances for multiple profiles, that’s also possible, but a bit of an advanced topic; it’s also not related to the OS, because it’s the same on all of them — only one profile is allowed at a time, by default.
There were some recent changes to profile handling in FF, which may make this easier; I haven’t checked those out yet.
Remove the ‘may’ from that sentence. Instead of having to use options from the command line to start the profile manager or select a profile you now have a menu on the menu bar and an option in the hamburger menu to do all of that. You get separate windows for each profile. And since theming is part of the profiles you can make those windows look very distinct from each other so the probability of mixing them up can be made very small.
Just to add to any confusion here, I’d also like to point out the possibility of having multiple parallel installs of your Firefox snap in case you want a stronger separation: