Alex, I just tested the Qt5 version of tomboy-ng on a Debian Bullseye running Plasma.
No problem copying accented characters and the Euro symbol from firefox to tomboy-ng, convential copy and paste and also the Unix style Primary Paste.
So I pulled down Cudatext, conventional copy and paste of both accented characters and Euro character, perfect (Cuda does not do Primary Paste ? You know, copy on selection, paste with middle button ?).
Anyway, clear that its not a problem on Debian Bulleseye copying from firefox to a Lazarus Qt5 app.
Davo