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.