This is a fork from https://github.com/Hubbitus/shell.scripts. I just changed it so that it works with Firefox Nightly instead of Firefox.
First, install Window Titler and set it up so that it adds [workspace-name] in front of window title.
Then, get the file and run.
git clone https://github.com/SinTan1729/Firefox-Nightly-Auto-Place-Windows-by-Workspace/
cd "Firefox-Nightly-Auto-Place-Windows-by-Workspace"
NightlyAutoPlaceWindow