It could very well be that the source of data (Firefox) only advertises the formats itself supports.
They way both clipboard and Drag & Drop work internally is that there is a “source” and a “target”.
The “source” makes an “offer” for its data and says which format it can provide. The “target” then chooses the one that fits best.
Quite often that is done in a way that the user is not even aware of, e.g. “target” picking an offer “format” that has the least loss of information.
The choice is then communicated to the “source” which then starts transferring the data.
In this case it appears that Dolphin delegates the choice of “format” to the user, since it is not really consuming the data itself but just writes it into the file.
It can literally paste anything that the “source” provides without needing any code to understand what a “format” actually means.