To find the correct #WMClass entry, in the terminal run "xprop WM_CLASS", it will show something like "Navigator" (common, Firefox uses it too) and "com.fushra.browser.alpha". This is the true entry that needs to be in the .desktop file so the system knows that the running application window should be grouped with the saved launcher.

Random note about #WMClass in #xorg: I'm trying out a binary of the Pulse Browser from their site, but it doesn't come with a .desktop file. Rather than edit the Firefox one, I'm using one from the Arch AUR.
It's great, except when I save a launcher in my panel the resulting window opens separately (hard to explain, I use icon-only launchers and end up with two icons). This is because the .desktop file has "StartupWMClass=pulse-browser" but the binary doesn't.

Random note about #WMClass in #xorg I'm using the binary from their site, but it doesn't come with a .desktop file. Rather than edit the Firefox one, I'm using one from the Arch AUR.