I have been having trouble for at least a couple of years. Three minutes after loading yahoo email, a popup message announces "No Internet Connection." Other tabs work fine, and my WiFi icon in the System Tray shows five bars. Besides the slowdown with Firefox, some emails have recently been arriving weeks after being sent. Yes, weeks.
At first I thought this was happening because my motherboard won't support Windows 10 and later. However, I've recently noticed several third party vendors have stopped supporting Firefox on their websites. So I have to wonder if Yahoo has secretly declared war on Firefox, because open-source coding won't hide code designed to steal information. Yahoo, since 2016, has openly bragged at how they share their servers with the government, and regardless of the current political bent in the White House, since several email providers allow messages to be searched for key words under the guise of national security. The trouble comes when less than scrupulous people get involved. Who watches the watchers? For more than ten years, some emails were quite late in arriving. Now it's three months in a row. A week ago, I checked for messages from someone I regularly correspond with. One message. Today, I suddenly have seven messages from that same person going back to November 1, and today is December 23! So, yeah, as the Bard once wrote, there is definitely something rotten in Denmark, and Firefox is the target.
