Original image upload didn't work. Here it is.

Does it only happen on that site?

Nope. It's not dependent on the site. It will happen on a blank tab page.

uninstall Firefox. Then Delete the Mozilla Firefox Folders in C:\Program Files and C:\Program Files(x86) Then restart system. Then run Windows Disk Cleanup. (Note: This should be Pinned and run Weekly, If never done below expect 10's of gig's) Then run it again and click the button that says Cleanup System Files. Note: your Firefox Profile is saved. But you should make a back up before you do : https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles

https://support.mozilla.org/en-US/kb/export-firefox-bookmarks-to-backup-or-transfer

Reinstall with Current Release Firefox 57.0.2 with a Full Version Installer https://www.mozilla.org/firefox/all/

Please let us know if this solved your issue or if need further assistance.

I have no doubt that would work, but it doesn't explain the behavior does it?

There was no where in your questions or answers that asked for a Explanation. Non Given.

Possibly a bad file in the update process. As a Support Volunteer am not given reasons or explanations nor am a Developer to know the answers to why for every question. I just help you fix the issue. I supplied one. Please let us know if that worked. If not there are others to try.

It did work, and I thank you. I didn't mean to be rude and I apologize for that.

Thank you for the help.