In the first place, I'm familiar with automated GUI testing. Thanks for the tips anyway :)
The reason I need this plugin (add-on) is not to start something new, but to re-use testscnario's that are in Fitnesse at the moment.
Regarding to the title: the scripts need to be 'converted' back to Selenium (IDE) format to be able to re-use it in another Selenium bases automation solution. So, it's to recover a set of a lot of testscenario's from Fitnesse that get lost otherwise.
So, the way to do this 'reverse' conversion is by uising the Firefox add-on 'Selenium Xebium Formatter'. I just checked with a recent Firefox installation and yes this Firefox Quantum (v61) is not able anymore to use this add-on. But I also have an old Firefox Portable version (31) where I was able to install this add-on by using the .xpi-file (because on my server I was not able to download it right away).
Anyway, Now I have to check if and how this add-on works (and how easy/difficult it is to convert the testsets back to Selenium)...