How to copy to clipboard from firefox dialog box

I have a Galaxy Tab 4. I'm using Firefox. I visit the "Safari" book site. I have no trouble paging through books on the web site. There is a "URL" button, which when clicked displays a dialog box with one read-only text field containing a URL representing the current page in the book.

At this point, I want the most convenient way to get this URL into the clipboard, so I can paste it somewhere.

I was able to get the text selected, although it's somewhat error-prone. It appears if I double-click on the text field, it will select all of the text of the field.

At this point, though, I don't know what to do. The text is selected, but I see no obvious way to get that text into the clipboard. I see no popup menu that looks like it's prompting me to save to the clipboard.

At this point, my screen looks like this: