This is a virus, not part of mozilla then do a virus scan and if it does not work reinstall firefox

It could be a message from the browser. Does your browser seem slow?

Start Firefox in Safe Mode {web Link} by holding down the <Shift>
(Mac=Options) key, and then starting Firefox. Is the problem still there?

See also:

• https://support.mozilla.org/en-US/kb/firefox-health-report-says-your-browser-is-slow

Can you attach a screenshot?

• https://en.wikipedia.org/wiki/Screenshot
• https://support.mozilla.org/kb/how-do-i-create-screenshot-my-problem

• Use a compressed image type like PNG or JPG to save the screenshot
• Make sure that you do not exceed the maximum size of 1 MB

Thank you all for the replies.

I've run a virus scan, and various malware scans (all of which I do regularly); no virus was found, and the suspicious bar has not reappeared so far (so I can't take a screenshot of it).

So, the suspicious bar issue may or may not be resolved; time will tell.

However, Firefox was extremely slow to load this morning; I'll keep an eye on it and try the other suggestions if the slowness continues.