Kostya, thank you for your prompt reply. If I understand you correctly, I have opened a tab by clicking on a URL in AquaMail and, when I click on another URL in AquaMail, it re-uses the same tab? If so, that makes no sense to me. I click on many URLs from email - I would never want them to overwrite an already open tab unless exactly the same - see discussion below. And in my case, the tab that was overwritten was one I originally opened in the browser, not from email.
If I understand your quote, I would expect AquaMail to go to an open tab if the identifier is exactly the same. That makes sense, e.g, I click on the same link to a book or news story or whatever. But if the identifier is not EXACTLY the same, I expect AquaMail to open in a new tab.
And even if I had originally opened a tab from AquaMail and then browsed to another site, that tab should not be reused by AquaMail because the identifier is no longer the same. As I mentioned in my original post, Gmail app does not behave this way.
Possibly it's a unique interaction between FireFox and AquaMail?
Would it help if I followed the instructions for creating a debug file and sent it to you? If the way AquaMail is working is the way you expect it to work, I will have to (ugh!) go back to Gmail app. I hope not.
Thanks for your help.