Or have a shortcut that has something like this as its target:
"C:\Program Files (x86)\Mozilla Firefox\firefox.exe" --ProfileManager --allow-downgrade -no-remote
This just opens the profile manager every time. The only caveat is that you have to click “launch” every time as there’s no timeout. But I also do have an autohotkey script that does the timeout for me, pressing “enter” after 30s.