After opening a site the page goes blank with a message "If page not displayed in (countdown from 5) seconds, click here." It always reopens without clicking.
This issue has only recently popped up, but seems to be more frequent.
You may have ad/mal-ware. Further information can be found in this article; https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware?cache=no
Run most or all of the listed malware scanners. Each works differently. If one program misses something, another may pick it up.