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?

Does toggling "Work Offline" mode make a difference ?

Thanks. I tried the first suggestion and am waiting for the problem to return, if it does.

I got the no connection message. Tried reloading the tab and it wouldn't go to email, like before. After about 5 minutes, the email site opened. I don't count this as solved.