Browser Issues
I'll be browsing a website, and with no rhyme or reason Firefox (on Windows PC) will decide that this website no longer exists and is no longer available, even the page I was on 2 minutes prior is no longer available (it even tried telling me that mozilla.org no longer existed). But the same site continues to work fine on the Firefox on my Android device.
The sites eventually "come back", but it's not very productive for a site to "disappear" in the middle of something.
Opaite Mbohovái (4)
It might have been a stale state which is unlikely unless you had the browser session open for a considerably long time. Were you able to verify the internet connectivity?
Browser would've been open for no longer than 10 minutes at the time of the issue. And when a site "disappears", other sites/tabs on the same system continue working just fine with no issues whatsoever.
Some add-ons may have been causing this. Can you help us with the add-ons you have been using?
Handful of ad-blockers (2 that are Facebook specific), a few reverse image searchers, and a couple of video downloaders. The issue only just started in the last couple weeks, and it happens so randomly, I can't (or don't know how to) recreate a scenario to test which add-on(s) could be causing it.