Great question — answering as the UphillTech team:
I have had this off-and-on for a few days (I'm running Fedora 31 beta for a couple weeks) and starting to cause issues with some of the sites I visit. Since Firefox has it's own list of certificates, I could verify it was not a network issue by pulling up Chrome/Chromium and view the pages. I was able to un-install then re-install Firefox and that has solved the problem for me.
Full answer by dan_linder: https://unix.stackexchange.com/a/549565 (CC BY-SA 4.0).
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