Its not just Evolution. Ive had the same problem in Thunderbird to FF
and Opera and Chrome. And the same from Opera (sometimes) to TB.

Yes, me too. It happens regularly with Thunderbird. The 'Clipboard',
appears to lose the copied item. Am using a rather clumsy work-around
with the Thunderbird screen size reduced slightly so that I can access the
FF desktop icon at all times to bring up FF.

This affliction started when I updated to Kubuntu 10.04 and has continued
with 10.10.

I can copy both ways FF to TB and vice versa with no problem with
Kubuntu 10.10 and Mint 10. I have never had a problem on any version
of Ubuntu or Kubuntu with the clipboard provided both applications are
open. Linux clears the clipboard when an application closes, however
the history in the drop down is saved and you can easily retrieve it.

Yes, I knew that, but on occasions the info' has been lost with
both apps still open.

I can live with it.

It is surprising what you can get used to. Just look at what Windows users
can put up with and things look rosier.

Try pressing the [ESC] key once then copy/paste again. I've seen that
work.

I have found if the document/application is not still active the data
in the cut is lost. For instance say I have a word type document with
a URL or just a name I want to google in Fire Fox and I scan it using
the mouse if the document is not still up the information is not pasted.
If I just reduce the document or bring the FF to the front screen then I
can paste the information into the search or other fields as desired.

This is not a copy & paste problem.

I have a LAMP set-up on my machine. In the past I built my pages,
filled them with text from Ooo and happily continued on my way to a
complete page. This however was disrupted some 6 months or so ago. The
problem was temporary however, a message popped up in Firefox telling
me Firefox had disallowed the action. I work off line, well using
Localhost but Firefox thinks I am On line without a local connection, so
I had to reproduce the effect while at my local hot spot. Firefox asked
if I would like to install the fix that seems to be more of an
intentional security thing, as I understand it. I said yes to the
inquiry, and I was then able to paste into the Firefox browser.

The reason I talk about my LAMP server is the fact that this whole thing
happened on my laptop rather than between two computers as it would be
over the net or something. The fact that it is a localhost connection
means little however, because we are using a web application so security
is handled the same way. Are you getting a request to install a firefox
pluggin as I did? I would guess you are. Please don't ask for more
info, it has been a while and for the life of me I can't remember what
the pluggin was called.