I don't think this is a Plasma issue. I use the Compose key a lot and have never seen this issue either in Firefox or in Falkon. I use Neon, so perhaps it is a Kubuntu issue; or perhaps you have assigned the compose function to a key, that browsers treat specially? Do you have the Plasma Integration addon installed in Firefox?
If you find out what the problem is, we welcome feedback so that we can update this page.
PS. Have you sees the other thread on this Talk page? It seems to be related, and a solution was found: scroll all the way down - you may have to click a link saying something like "Show 4 answers" to see the last entries.