dwm is supposed to put Firefox windows on the ninth tab.

/* xprop(1):
 *	WM_CLASS(STRING) = instance, class
 *	WM_NAME(STRING) = title
 */
/* class      instance    title       tags mask     isfloating   monitor */
{ "Firefox",  NULL,       NULL,       1 << 8,       0,           -1 },

However, recent firefox versions report this in xprop:

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

Now the firefox windows show up anywhere.

chrome shows up on tab 9 as expected.