First, let's make sure your system is clean.

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.

Many site issues can be caused by corrupt cookies or cache.

Warning ! ! This will log you out of sites you're logged in to. You may also lose any settings for that website.

• Delete cookies to remove the information websites have stored on your computer {web link}

• How to clear the Firefox cache {web link}

If there is still a problem, 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). Did this help?

While you are in safe mode;

Try disabling graphics hardware acceleration in Firefox. Since this feature was added to Firefox it has gradually improved but there are still a few glitches.

How to disable Hardware Acceleration {web link}

Thanks. I think some program on my mac is trying to exploit Firefox Accessibility Services to perform a sneaky clickjack. I enabled "Prevent accessibility services to access your browser" from Preferences under Privacy and Security. This is now makes sure the clickjack does not work.

I have installed Malwarebytes (free) and Avast (free) but both do not report anything unusual.

I would like to know which program is accessing this feature. How can I find this? Please let me know.

werty37 said

I would like to know which program is accessing this feature. How can I find this? Please let me know.

This should be written on the about:support page, in the Accessibility section. Parhaps you have to disable prevention to see the activator.

Thank you. It does not seems to happen anymore after I installed Malwarebytes, thought it did not find any suspicious activity.