Our organization has several RTC instances, some are 6.0.3, others 6.0.2, but I have experienced this issue on all of them, and with different browser versions for several years of using RTC web ui client regularly.
I currently have FireFox ESR 45.8.0, but as I said I've seen this for quite a while on different versions of FireFox and RTC.
The issue seems to be intermittent, it usually happens after I have already been using the RTC WebUI for sometime for reading work items and updating them with comments. For no apparent reason, suddenly the Copy and Paste functionality apparently stops working.
If I click the mouse on the Add a Comment field of the work item, and try to paste something from my clipboard (doesn't seem to matter from where, whether from another application, or even text in another comment on the same work item page). If I try to use the same copied data in the clipboard I can usually paste it fine into other applications like a text editor, but not to the comments in the browser.
This happens to me on average once a day using the WebUI client, so I often have to open the same work item in another browser (Chrome) or Eclipse with RTC plugin to do my work. I have not been able to diagnose any particular action that triggers this behavior. I tried looking at the Developer Tools for any console errors, and did not see any.
I have never seen this issue with Chrome, although my default browser is FireFox so would like to understand if something can be done on my end.
Comments
Does restarting Firefox fix things? My observation is that Firefox keeps things in the memory for a long time (probably for ever) - for example, if I open and close many web pages, Firefox ends up with consuming several GB memory even though only one tab is displayed at the end. This can surely cause performance or other unexpected behaviors.