Firefox already has built-in a way to select multiple tabs and select all tabs. Literally you can right-click on a tab > "Select all tabs".
You could just add to that same menu the option: "copy" to copy into the clipboard a list of URLs that of the selected tabs.
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@fcolecumberriI really like the idea, but could you edit the title?
Maybe to "Allow to copy selected tabs urls (to clipboard)"?
By the way, the Android version of Firefox can already do this, but the desktop version not.