This bug was first reported on 2021-12-06 with KDE Plasma 5.23.4 and there is still no fix in version 5.27.3!

https://bugs.kde.org/show_bug.cgi?id=446581

The freeze is presumably caused by Firefox attempting to update the clipboard and show a notification at the same time.

A temporary solution would be to disable Firefox notifications:

https://support.mozilla.org/en-US/questions/1283277
https://support.mozilla.org/en-US/questions/1345694