Same issue started in Aug 2022 for me and is still happening now in Dec 2022. Once in a while I will get a pop up window asking if I want to stay or leave the page, but for the most part it just shuts down the FF browser completely. I have emailed yahoo support and their latest suggestion was to try a private window and disable all extensions. Neither fixed the issue. Since I use my email for work and open numerous pdfs throughout the day, I have had to revert to Google Chrome browser for any emails with pdfs. BTW, this also happens if I attempt to open video .mpeg files. Would love to know how to fix this so I can stick with the Firefox browser. I hate having to use google.

This was yahoo's first response back in Aug (none of these steps fixed the problem):

Please follow the troubleshooting steps outlined in the email. After you complete each step, please test to see if the issue is still present.

Use these steps, starting with #1, then check if your problem is resolved before moving to the next step.

Clear your browser's cache. Try to use a different web browser. Update your browser. Restart your computer. Make sure that JavaScript is enabled. Disable browser enhancements. Temporarily disable antivirus, antispyware, and firewall products. See if the Yahoo service works from another computer or mobile device. Reset your browser to default settings.

If the issue still persists, please provide as much information as you can in your next reply.

Describe the steps you take leading up to the problem. How are you accessing mail when the problem occurs? Through your device’s Internet browser, through its default mail application, or through the Yahoo Mail app? How often does this issue occur? Are you getting an error message? If so, what does it say? Can you replicate this problem from a computer and/or mobile mail app? Please take a screenshot of your computer screen and attach it to your reply to this message.

And this was their last response in Sept (they haven't addressed this issue since):

The engineers have requested be done on your end to try and resolve the issue. You are using the latest version of the Firefox browser 104, can you try using a private window in Firefox: https://support.mozilla.org/en-US/kb/private-browsing-use-firefox-without-history

Also, can you please check the settings you have for PDF files: https://support.mozilla.org/en-US/kb/view-pdf-files-firefox-or-choose-another-viewer

Please let me know the results when you try these steps. Thank you for your patience.

Have a great day. Regards, Natalie Yahoo Customer Care

Are you still experiencing this problem with Yahoo mail closing? This is a copy/paste of my comment in another thread today (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.