This is basically a Firefox issue and it's quite difficult to change the behavior. I am looking at partial workarounds for specific scenarios, but they're very much not straightforward and they end up being mooted if you move the cursor around. Strictly speaking, what FF is doing isn't wrong; it's just ambiguous. If you have a cursor at the end of a link, is it inside or outside the link? Both interpretations are correct. Chrome takes the view that you're outside; the effect of this means that you can't actually extend the link from the end (you have to move back a character). FF takes the view you're inside; the effect meaning that you can't write text immediately after the link (unless you can move into it).
Just for a point of note, I have managed to trigger this in each of the "major" RTEs as well; it's not exclusive to XF.