Not sure why, or if i'm the only one, but sometimes I have this problem with Firefox (latest) on Win10:

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.