Copying from Konsole to another application like LibreOffice or Firefox should paste text without formatting. The current behaviour is to paste the text with formatting (using the font, bg color and fg color from the current Konsole profile), which is very annoying and completely useless. I cannot imagine a situation where I would want to copy the particular formatting of my terminal to a document that I am preparing. The default behaviour should be to copy just the text. The current behaviour could be offered as an option (in case there are users that actually want this) or just removed.
Make sure "configure current profile" -> mouse -> "copy text as html" is disabled.
Thank you very much for you answer. You are right: changing that preference fixes the problem. I did not expect to find a preference for that under the «Mouse» section, because copying can be done using the menubar, the keyboard or the mouse, not only with the mouse. I did not look there. And I only created the bug report after searching the web for a solution and seeing other people with the same problem and no solution provided. So, at least there is currently an usability problem (discoverability). I think that the default for this option should be changed and the option should be moved (maybe to the General or Advanced sections). This is because, although I can see the usefulness of copying the format in a few very specific cases, in general it creates problems more often than not. Thanks again for your help and your work.
you are right, konsole settings *needs* a overhault. we are working torwards that too.
Right, it is tricky categorising the settings; the rationale behind it being in "Mouse" is that usually you select text in terminal using the mouse :) I agree though that discoverability is an issue.