Window Placement not working for Firefox

[roots]

Hi,

the Place Windows plugin is not working with Firefox since some Firefox versions back, that is, roughly a year or so.

Expected behavior: When configuring fixed window placement for Firefox on a certain viewport (e.g. (3,2)), Firefox is placed on the selected viewport upon application start.

Observed behavior: When configuring fixed window placement for Firefox on a certain viewport (e.g. (3,2)), the first instance of Firefox will always open on viewport (1,1), no matter if I query for window class, title, name etc. However, if I open a second instance of Firefox, it will appear on the designated viewport.

This is currently observed with various environments:

Ubuntu 20.04.2 LTS
Xfce 4.14, Mate 1.24, Gnome 3.36
Compiz 1:0.9.14.1+20.04.20200211-0ubuntu1
Firefox 86.0+build3-0ubuntu0.20.04.1

All other apps place correctly with the Place Windows Plugin.

Any suggestions much appreciated!