Your problem is beginning to look like Firefox – which has some history of failing to deal with HTML correctly over the last year or so. See these for example
https://support.mozilla.org/en-US/questions/1201465
https://support.mozilla.org/en-US/kb/fix-problems-images-not-showhttps://support.mozilla.org/en-US/k...
https://support.mozilla.org/en-US/kb/websites-look-wrong-or-appear-differently
I note that version 60.0 released in May 2018 contained this “fix” also -
Redesigned Cookies and Site Storage section in Preferences for greater clarity and control of first- and third-party cookies
I wonder if that’s working correctly
So, which version are you using? Current release seems to be 60.4.
The only way to determine if it is all down to Firefox is to use a different browser for a trial period. I suggest you download Chrome and report back if that does the same thing.
Please also confirm that you are not using Adblock or any similar ad blocker.
On the speed issue – what nominal speed is your connection -100Mbps?
Suggest you check the speed via Ookla speedtest (and NOT via wifi – use a wired connection from laptop to the router. Speed can drop dramatically over wifi).