I have the same issue on openSUSE Leap 15 - it's not Windows specific. I've found that the number of tabs doesn't matter too much - if I copy the URL of the non-loading tab, close the tab, open a new one and paste the URL there, it will load around 9/10 times (if it doesn't, third time is always the charm).
Drove me nuts enough that I switched to Brave for a bit as my main driver, but I've been experiencing some crashing from it recently and I miss my bookmark keywords, so I might go back.
When I open a new tab, there is a ~50% chance that the URL i'm querying won't ever connect. As if there was no internet.
I tried to remove some open tabs (I have ~20 open tabs all the time) and it worked again.
When it happens, opening a new tab on Chrome always works, though. I wasn't using a proxy and my internet connexion is pretty stable and fast.
Restarting Firefox also helped, IIRC. But I never had such a problem with Chrome, when it was my main browser. Now, this happens every month or so with Firefox.