how to start a firefox with a different WM_CLASS? (or any other altered property like WM_NAME)

i'm on mint linux with i3wm

i want to discriminate a special firefox session from the default sessions when its starts. So i need it to have at least one altered property right at the beginning (later it changes its title but then it is to late. I want to assign it via an i3 command to a special workspace)

i tried this

firefox --class="pidgin"
#xprop output: WM_CLASS(STRING) = "Navigator", "Firefox"
firefox --class=pidgin
#xprop output: WM_CLASS(STRING) = "Navigator", "Firefox"

this works:

xprop -set WM_NAME "Horst" #this works but its not at startup
#xprop output: WM_NAME(STRING) = "Horst"

this dosn't:

xprop -set WM_CLASS "Pidgin"
xprop: error: unsupported conversion for WM_CLASS

1) how do i check that the firefox --class=<1> command works?

2) what is a valid value for <1>?

3) is there any other way/command to start firefox with an altered property?

Solution 1:

To address 2), any string does the job:

$ firefox --class foo

i3 assign catches it (which is enough for me), but xprop still shows the old value.