What is the warning?

Start Firefox in Safe Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration or userChrome.css is causing the problem.

• switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Themes
• do NOT click the "Refresh Firefox" button on the Safe Mode start window

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

The warning is, "A web page is slowing down your browser" followed by two choices: Stop or Wait and a bright yellow bar across the top of the active window. NEITHER "stop" NOR "wait" is reasonable on a multi-tasking desktop PC. The message doesn't even bother to identify *which* page triggered the all-too-ubiquitous yellow-bar "warning."

Firefox is the ONLY browser that exhibits this. Happens multiple times a day. I'm getting fed up with Mozilla's failure to fix Firefox-specific BUGS. This bug has been present for YEARS and 99% of the time, I only use the "default" theme.

”Webpage is slowing down your browser” This is the new way the browser tells you there is a problem with the script program(s). https://support.mozilla.org/en-US/kb/warning-unresponsive-script?cache=no

https://support.mozilla.org/en-US/kb/task-manager-tabs-or-extensions-are-slowing-firefox