Even from before it was called Firefox, Mozilla has been a bloated, misdesigned, user-hostile disaster.
Mozilla has never been able to work properly on X11 without a window manager, and more precisely a reparenting, click-to-focus one. Any focus-follows-mouse wm (or any innovative UI) had to implement ugly kludges and special cases just for being able to click links, open menus or enter data into forms.
Mozilla has never implemented the selection protocol protocol properly, whether for the clipboard (the one that is pasted with Ctrl-V), the primary selection (the one pasted with the middle-click) or more complex stuff, like dragging-and-dropping. Mozilla had always assumed that it could fetch the same selection multiple times, making impossible to implement e.g. something like "... | xsel -write" without buffering first the entire data.
Any program or library had to do that stuff in the same non-standard, degraded manner just to be able to interoperate with mozilla (and other similar crap, like java or openoffice).
I don't think that anybody at mozilla has ever bothered in the last 20 or 30 years to spend the 20 minutes needed to read the spec and understand how that stuff works: they simply assume that the clipboard is some kind of buffer, and if doesn't work like one it has to be "broken". Their ignorance and incompetence has also led to gaping security holes, like any visited site being able to write to the primary selection via javascript without any user interaction.
It's no wonder that the stupid, arrogant cocksuckers can't fix their garbage, so all they can do is go on and remove features, because supposedly "nobody" is using them.