You're certainly trying to do things the hard way! I normally just send
the link from the browser. Firefox has a "send link" button you can click.

Just out of curiosity, why close firefox to send email? Linux is
perfect for running multiple applications - keep each one on its own
desktop, and switch as needed.

Not related to sending links, since that's a solved problem, but one
issue with copy and paste on linux is that there are different
mechanisms at work. In the old days it was easy: middle button copy,
left button paste, worked every time. Now it seems that some non-native
style copy and paste methods have been grafted on, the problem being
that some applications only use the external "clipboard" buffers and
ignore the native unix copy/paste buffers.

you can download klipper:
sudo apt-get install klipper

Just don't close Firefox before you have pasted. Is that so hard?

It is not Windows, wasn't that the whole idea?
Don't expect them to be the same...

Suggestion, don't close the browser. Even Microsoft drops the clipboard contents
if you close the browser, pre-Vista anyway.