Don't set wm_class of "Navigator" when starting Firefox

I'm starting Firefox as follows:

/usr/bin/firefox --no-remote -P Fastmail --class "FastmailFF"

The ultimate goal is to figure out a command to put in a .desktop file which will have it's own icon and exist separately to the normal browser. The above gets grouped under the current Firefox icon. When I inspect the window using xprop WM_CLASS command, I get the following back for the above:

WM_CLASS(STRING) = "Navigator", "FastmailFF"

and the following for the main Firefox:

WM_CLASS(STRING) = "Navigator", "firefox"

So I figure the common "Navigator" string is causing the grouping. Is there a way to prevent Navigator getting added upon startup? It looks like firefox is installed via a snap:

Version: 1:1snap1-0ubuntu2

I'm running gnome 42.5 and wayland