I'm just starting playing with Python Selenium (for working with Adobe FlashPlayer ActionScript3 stuff that's now obsolete) & as my second script, got Selenium to successfully open WaterFox.
driver = webdriver.Firefox()
```
with
```
driver = webdriver.Firefox(firefox_binary = "/my/path/to/waterfox")
```
It started WaterFox, then closed - just like it did with FireFox
(Ubuntu 20.04, Python 3.9.5)