From the bug report:

[...] will require moving some config & data files from ~/.phoenix to probably $XDG_CONFIG_HOME/phoenix and [...]

According to Wikipedia, the browser was already renamed to "Firefox" when that bug report was opened, but still wild to see that even back then, they already had some technical debt for the name of that directory.

Unfortunate that they did not go straight from ~/.phoenix to $XDG_CONFIG_HOME/mozilla, but hard to say, if that was as obvious of a choice back then...