This is with latest released Nightly. I managed to figure out that the broken state of my Firefox (no website able to load) was caused by Disconnect addon presence.

Problem is that this addon is installed for a very very long time, and its last update is from July 2019, so it's unlikely anything changed here, which makes me think to a regression in Firefox and some addon APIs used by that addon.

This also affects with Vimium-FF addon, this was happened after updating to Firefox Nightly 78.0a1 build: 20200519160549.

I have both Disconnect and Vimium-FF addon installed and last updated on April 03, 2020.

This should be a duplicate of Bug 1639478 (which contains a bit more details about what is going on), but it would be good to also double-check if there are other conditions (besides the extension injecting a CSS as described in Bug 1639478) that may be triggering the issue.