In the past, there had been a split between Debian / Linux Devs, and the Mozilla team, where at first Linux was allowed to share “Firefox”. According to that split, Debian / Linux continued to develop its own version of the Web-browser, naming that “
Iceweasel“. Newly, this split seems to have been resolved, so that Debian is now offering Firefox again. The packages have been made available.
Just now, I did my own transition, from the deprecated Iceweasel to Firefox. This process was quick and painless.