I am using Slackware64 14.2 and Thunderbird 52.1.1.

Here is the problem, when I compose a message, I sometimes highlight some text and insert a link. If I past the link using the middle mouse button in the Link dialog box, the following problem occurs (typing the link in or pasting by other means, i.e. the windows way, does not cause the problem). The recipient of the email clicks on the highlighted text expecting the link to open in their browser but nothing happens.

However, if I change View - Message Body As - Original HTML to View - Message Body As - Plain Text, in the received TB, the link appears under the text and clicking on it opens it in my browser.

What is really confusing is the source html in the receiving TB is identical whether the link was pasted with the middle mouse button or entered any other way but the link doesn't work if the link was middle mouse pasted?

I am posting this for informational purposes in case somebody else runs across the same problem.

Copy by selecting and paste by middle mouse click is traditional X-Windows way. Copy to buffer by Ctrl-C and paste with Ctrl-V is different. Someone told me that those two are handled by different protocols.

It also depends on the source where are you pasting from. Sometimes you can't paste something into terminal, but you can paste into graphical editor. like Leafpad.

Pardon me, but I can't avoid doubt that two HTML sources are exactly the same. Perhaps there is some additional quoting, escaping, ampersand characters, or characters codes preceded with '%'.

^ i'd rather guess there are difference in the (html or email) headers.