Firefox has way too slow DNS resolution
The initial handshake takes typically a couple seconds in Firefox, while it takes in Chrome or Safari typically < 1s to display the first content.
Here an example of opening firefox.org: https://share.firefox.dev/3jpZRG4
I've tried all the recommended procedures without any success (personal mode, disabling extensions, starting with clean profile, ...)
Even this form I had to send twice because there was a timeout when sending the data up.
My internet connection is always > 30Mbps, so that should not be a problem.
ప్రత్యుత్తరాలన్నీ (2)
You can check the Network Monitor to see if content is blocked or otherwise fails to load.
If necessary use "Ctrl+F5" or "Ctrl+Shift+R" (Mac: Command+Shift+R) to reload the page and bypass the cache to generate a fresh log.
You can try to disable IPv6 (check for other possible causes as well).
The other browsers have a faster fallback to IPv4 so it is possible that his IPv6 DNS is not configured correctly. That is what I would check first. Simply turning off IPv6 without checking out the problem is like cooking all your food in a microwave because you have problems operating the stove...