I wonder whether this is related to a problem with users losing Google connections and getting Oops errors in Gmail in the middle of their session. Here's the temporary workaround for that, in case it helps with Yahoo, too:

Turn off the "zero round trip resumption" feature -- this is intended to speed up active website connections but doesn't seem fully compatible with some sites. Here's how:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk.

More info on about:config: Configuration Editor for Firefox. The moderators would like us to remind you that changes made through this back door aren't fully supported and aren't guaranteed to continue working in the future.

(2) In the search box in the page, type or paste security.tls.enable_0rtt_data and pause while the list is filtered

(3) Double-click the preference to switch the value from true to false

After that, try the site again (either reload or reopen in a new tab). Any improvement?

Sadly not resolved the issue for me. I had no Google issues as you've indicated were possible anyway. If i access Sky/Yahoo via Google all my emails are available to me right back to 2016. The problem is still with Firefox unfortunately. I have tried refreshing Firefox as asi read somewhere on Mozilla pages but also to no avail. I have cleared my cache & again it remains an unresolved issue. Thanks for trying though.

You could check Firefox's "Web Console" to see whether any of the error messages listed there shed light on the problem. Here's how I suggest diving in:

(1) Load the Inbox but don't scroll down yet.

(2) Open the Web Console in the lower part of the tab using Ctrl+Shift+K

(3) (In the console) Click the tiny trash can above the list to clear it.

(4) (In your mailbox) Scroll down until the problem occurs and allow up to 60 seconds for error messages to appear. Anything?

Because this problem just started recently:

Firefox recently rolled out "Total Cookie Protection" to more users. This limits how sites can use "third party" cookies, meaning cookies from different websites than the one you see in the address bar. Yahoo probably uses multiple servers for mail, and there could be an issue with cross-site sharing of cookies.

You can make exceptions to this feature using the controls for Enhanced Tracking Protection. Click the shield icon at the left end of the address bar to drop down the info panel, then click the slider switch to turn Enhanced Tracking Protection off on your Inbox. You may need to repeat that on the page where you log in to your Yahoo account. More info in the following articles:

• Introducing Total Cookie Protection in Standard Mode
• Total Cookie Protection and website breakage FAQ