I just got a specific configuration that is working so far! Don't know if it's going to have problems along the road but it may be useful to other people.
It's hard to make it work because you need to have a specific Firefox, Geckodriver AND selenium configuration! So how did I check which versions should be used? I started with geckodriver.
They have a changelog but it's not in all releases that they give an insight of which versions of FIrefox/selenium are compatible or not. Since I'm stuck in Firefox 52.6.0 ESR, I searched for "Firefox 53" in their full changelog. In version v0.18.0, there's geckodriver now recommends Firefox 53 and greater so I downloaded v0.17.0 directly into my /usr/bin folder:
sudo wget -c https://github.com/mozilla/geckodriver/releases/download/v0.17.0/geckodriver-v0.17.0-linux64.tar.gz -O - | sudo tar -xz -C /usr/local/bin/
Since they don't tell which Selenium version is compatible with gecko 0.17.0, I kept reading the changelog until I found Note that geckodriver v0.16.0 is only compatible with Selenium 3.4 and greater. and decided to use this version as well.
So, for now, with latest Firefox ESR 52.6.0, these pins are working:
Plone
4.3.15
Firefox
52.6.0
Geckodriver
v0.17.0
Selenium
3.4
The "selenium.common.exceptions.WebDriverException: Message: Can’t load the profile." error is now gone. I don't know if pinning selenium to 3.4 will have bad consequences in really complex test configurations. Don't know if it's possible to automate the geckdriver download in a buildout since I'm using sudo but it would be nice as well.
FYI @gforcada and @mauritsvanrees, since the github issues and commits that I posted here were from you.