Can someone just confirm that FF and FFdev can co-exist happily on (Ubuntu) Linux with FFdev causing no side effects on FF?
Yes, I have Firefox Release (by Ubuntu) and Dev Edition and Nightly all together running on Ubuntu. Just make sure you install Dev Edition from https://www.mozilla.org/de/firefox/developer/ (I would recommend against any other packages source in this case, the Mozilla bundles work very good and also handle updates very well. I found external ppa or ubuntu-make to be outdated or not working equally out-of-the-box).
In case you would install Nightly then you would need to create an extra profile. But Dev Edition comes with its own profile by default - so, no worries