Below link may shed some light on gtk3 controversy.

https://igurublog.wordpress.com/2012/11/05/gnome-et-al-rotti...

I don't have a definitive answer, but I think it'd be from association with Gnome 3, which draws out all sorts of pointless and tiresome flame wars amongst people who thrive on creating (and then wading into) emotionally charged drama about software, of all things. :(

Good question.

Suppose I have a KDE based desktop which I gather uses qt as the widget library. Am I going to have another subset of libraries to run Firefox?

It is the new theming engine in gtk3 which does not seem mature (or maybe there just are no good skins yet).

Gtk+ 3 is part of the modern degenerate conspiracy along with systemd and other abominations such as kdbus to destroy Linux by straying from the time-honored ancestral traditions of UNIX which were devised when men were still true hardworking men, that spent hours of sweat and tears configuring their window managers and compiling kernel modules as was ordained by the ancients.