Is there any way to copy the tooltip text in Firefox?
No. Of course, you can ctrl-u and search for the tooltip in the html source or you can make a screen shot, but I guess that's not what you're after.
I've seen this trick working with tooltips on some websites - select tooltip & its surrounding words, copy selected text while the tooltip text is visible & then paste at the desired location. You'll need to remove the extra words you copied initially.
When hovering over text open Firefox context menu by right clicking:
inspect element > (right click on highlighted text) copy > inner HTML