antivirus?
I also use Firefox and don't have the issue. Firefox 78.0.2.
Rich I wonder if it is your antivirus? Some AVs create a secure browser link and I think they route traffic to secure / logged in websites via their own servers.
I know someone who recently changed to kaspersky AV and logged in to their internet banking using firefox, and did the online banking and logged out, all as normal. A little while later their bank sent them a suspicious activity warning, saying the account had been accessed from Russia. The access was actually from Australia, about as far from Russia as you can get. The time listed in the warning email matched the time of the legitimate access from Australia. the person asked me what I thought was going on, all I could think of was the recently installed Kaspersky, which does list securing your browser as one of its features, may have routed the traffic via its own routers in Russia. If this takes traffic via a congested server, you would get the slow response.
I freely admit I am out of my depth here, I could be talking BS without meaning it. Seek further advice.