I use Tab Mix Plus :: Add-ons für Firefox - Mozilla Add-ons, which can do that and much more:
Context menu of Tab Mix Plus (click image for larger variant)
As you can see in the screenshot, I'm on the right-most tab, while I opened the context menu of the left-most tab. Using the highlighted menu entry, I get the URL of the latter copied to the clipboard.
Tab Mix Plus can do a lot more things for you, which I'm not going to explain in detail here (as you didn't ask for, and the list would get quite long). Some you can already see from the screenshot:
- duplicate/protect tabs
- optionally open links supposed to go to separate windows in new tabs instead
- suppress popups
- scroll or stack tabs
- close/re-open tabs, or "close all tabs left/right of this"
I'm using TMP for years already, and it's always one of the first addons installed on a new machine – could not do without. Dev is very responsive and helpful, development steady. In short: heartly recommended :)