So where are the download hosted at? And did you uninstall any previous firefox before installing this version?

Start Firefox in Safe Mode {web link} by holding down the <Shift> (Mac=Options) key, and then starting Firefox.

A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?

It is possible that security software (anti-virus, firewall) is causing the problem. Try to disable security software temporarily to see if that makes a difference.

Could be because:

"Added content blocking, a collection of Firefox settings that offer users greater control over technology that can track them around the web. In 63, users can opt to block third-party tracking cookies or block all trackers and create exceptions for trusted sites that don’t work correctly with content blocking enabled." - FF 63 Release Notes

• https://www.mozilla.org/en-US/firefox/63.0/releasenotes/

~Pj

The downloads are from various different sites. I did not uninstall the previous version before installing this version. I just did an update. Should I have uninstalled the previous version?

Woodcock said

Should I have uninstalled the previous version?

If you updated, or got the full installer, it would have replaced the current program.

cor-el said

It is possible that security software (anti-virus, firewall) is causing the problem. Try to disable security software temporarily to see if that makes a difference.

I tried this and it did not make any difference.

FredMcD said

Start In Safe Mode (not Refresh). Is the problem still there?

I tried safe mode and it did not make any difference.

Woodcock said

FredMcD said

Start In Safe Mode (not Refresh). Is the problem still there?

I tried safe mode and it did not make any difference.

Did you also try in PC Safe Mode with Networking? And did this produce anything different?