Are you still experiencing this problem with Yahoo mail closing when you download (not preview) an attachment? There are numerous threads about that. This is a copy/paste of my comment from another thread which might be relevant to your issue (from https://support.mozilla.org/en-US/questions/1401782#answer-1558480):

There is something unusual going on, but first, this seems to work normally:

(1) Open a new tab and open your Yahoo Inbox in only that one tab (2) Click the message that has the attachment; it appears in the same tab (3) Hover the image attachment and click the download button and Save the file -- nothing weird happens

If I force Yahoo Mail to display a message in a second tab, and then download an attachment from that message, then I am able to replicate the unwanted behavior. Here are the four ways I tested:

Scenario A: Open Inbox in a second tab or a new window. Either:

• right-click the yahoo!mail logo and choose Open Link in New Tab
• right-click the yahoo!mail logo and choose Open Link in New Window

In this scenario, I can open and view a message in the new tab/window without any rude surprises, but if I open a message and download an attachment, then at some point thereafter, the new tab/window is closed.

Scenario B: Open a single message in a second tab or a new window. Either:

• Shift+right-click the message in the Inbox and click Open Link in New Tab
• Shift+right-click the message in the Inbox and click Open Link in New Window

Same here: I can view the message without any rude surprises, but if I download an attachment, then at some point thereafter, the new tab/window is closed.

Can you tell whether you have Yahoo mail open in a different tab? If you type or paste %yahoo in the address bar, Firefox should list all the open Yahoo tabs. You could scan the list to see whether there is another mail tab open which may be the master tab that stays open.

How to allow a second Yahoo! Mail tab to stay open?

I'm doubtful that Firefox is deciding to close the tab for its own reasons because (A) there is an inexplicable delay after the download is completed and (B) it doesn't happen on other sites.

But how could Yahoo! control this? Perhaps a script in the page itself is closing the tab when it was launched from another Yahoo tab. To test that, I tried these methods to open the second window/tab in a completely independent way:

• press Ctrl+T to launch a new tab, type mail.yahoo.com in the address bar and press Enter to load it (Ctrl+clicking a bookmark should also work)
• press Ctrl+N to launch a new window, type mail.yahoo.com in the address bar and press Enter to load it (Shift+clicking a bookmark should also work)

Neither of those got closed after opening a message and downloading an attachment.

That's what I've discovered so far. It's not satisfactory that Yahoo is allowed to close a tab when it wants to. Normally, scripts in pages were only allowed to close their tab if the tab was launched using a script. Since the links were launched in a second tab/window by me using Firefox's link context menu, I didn't think the page would be allowed to close its tab in that case. Something seems to have changed.

Thanks for investigating this. Yes, still having this problem. I only ever have one Firefox tab open for Yahoo mail. I replicated what you did in scenario A, and the Firefox mail tab stayed open, go figure.

The issue does seem to happen more often when I try to download a document even from within Yahoo mail, and not only when I preview it in another Firefox tab. Note also that if I have multiple message tabs open in Yahoo, they sometimes close at random too. My Firefox Yahoo mail tab also sometimes closes at random even when I haven't downloaded anything.

I also tried using Firefox in safe mode with plugins disabled and the tabs still close.

I think any tab opened using a link (especially a link from another Yahoo page) is at risk of getting closed whenever a Yahoo script wants to close it. Tabs you open manually (for example, by typing mail.yahoo.com in the address bar and pressing Alt+Enter) seem immune. However, I only use Yahoo mail a few times a year, so I'm sure I haven't seen all of the ways in which it can misbehave.

After looking into it for quite a while, I don't understand why it only seems to happen in Firefox -- is it that Yahoo is designed to work differently in Firefox, or that other browsers disregard the instruction to close the tab?