hello, sometimes such issues are caused by a firewall/security software which doesn't recognize & therefore blocks new firefox versions: Fix problems connecting to websites after updating Firefox

That's a possibility; we installed Avast security on the computer last week. I tried turning it off, though, and that doesn't seem to fix the problem.

I also tried turning off Windows Firewall, but again, it didn't fix the problem.

in this case please also try deleting all program rules for firefox within avast security & let it detect the browser again...

Note that sometimes you only disable the user interface of security software, but the firewall or anti-virus services are still running in the background.

After restarting Windows, Firefox is working again. I don't know whether something was updated over the weekend, or whether the restart itself was responsible, but either way I appreciate your help. Thanks!