A major GTK+ 3 update landed in Rawhide recently, and there are at least a couple of fairly prominent bugs related to it. Lots of text in the Firefox interface – on tab titles, menus, buttons, and things – is now white on grey, which is ugly-to-borderline-unreadable. Also, GNOME terminal windows are now extremely prone to shrinking when focused or unfocused, or when you create and close tabs.
Both bugs have been reported and should be addressed soon:
so you may want to copy yourself on those bug reports to follow progress. In the meantime, you might want to drop back to GTK+ 3.15.3, or perhaps just switch to a different Terminal app and live with the white text in Firefox.