hi, can you capture a performance profile once that happens and share it here? https://developer.mozilla.org/en-US/docs/Mozilla/Performance/Reporting_a_Performance_Problem

https://perfht.ml/2o5Broi

thanks, unfortunately it's the avast security software on your system that is interfering and slowing down js execution in firefox in this case...

I have already tried temporarily turning off AVG but the problem remained.

no, the performance profile shows that avast security software (not avg is interfering) in your case...

Is there a work around for this?

i am not familiar with avast - their module causing the issue is dubbed "aswJSFlt.dll" (so probably related to js filtering). you can try to disable script scanning in their webshield settings until they come up with an update that fixes this problem: https://support.avast.com/en-gb/article/167/

I don't have any avast software installed. Nor do I remember installing it.

oh, they seem to belong to the same company now - so probably avg is using avast components now as well. avg also has various options related to script scanning apparently: https://help.avg.com/en/avg_free/17/settings_sh_web_basic.html

if these don't work, try to remove it altogether for testing purposes once.

I have already tried temporarily disabling avg including the webshield

disabling may not be enough - try to uninstall it

That seems to have solved the issue.

thanks for testing. we are also in contact with the security vendor, so hopefully they will come up with a fix soon. we're tracking the issue in https://bugzilla.mozilla.org/show_bug.cgi?id=1438829