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).

Is the problem still there?

https://support.mozilla.org/en-US/kb/websites-look-wrong-or-appear-differently

https://support.mozilla.org/en-US/kb/firefox-slow-how-make-it-faster

https://support.mozilla.org/en-US/kb/firefox-uses-too-many-cpu-resources-how-fix

https://support.mozilla.org/en-US/kb/firefox-hangs-or-not-responding

https://support.mozilla.org/kb/Firefox+is+already+running+but+is+not+responding

I have run the suggestion on starting Firefox in Safe Mode and found that despite a couple of websites (e.g. CNN and SCMP) start up a bit slow, the browser does not appear to run sluggish - in comparison to running Firefox normally.

It could be the work of one of your add-ons. Type about:addons<enter> in the address bar to open your Add-ons Manager. Hot key; <Control>(Mac=<Command>)<Shift> A)

In the Add-ons Manager, on the left, select Extensions. Disable a few add-ons, then Restart Firefox.

Some added toolbar and anti-virus add-ons are known to cause Firefox issues. Disable All of them.

If the problem continues, disable some more (restarting FF). Continue until the problem is gone. After, you know what group is causing the issue. Re-enable the last group ONE AT A TIME (restarting FF) until the problem returns.

Once you think you found the problem, disable that and re-enable all the others, then restart again. Let us know who the suspect is, detective.

I have checked out my Extensions from the Add-On Manager and found that most of them are based on the Test Pilot I am participating in - with the exception of AdBlock Plus. I have essentially disabled that function and found my Firefox browser's performance has improved since then.

I will monitor the situation further and if needed, I may disable more Extensions (or in some cases, close some Tabs just in case they are having an impact on my browsing experience).

https://testpilot.firefox.com/ https://support.mozilla.org/en-US/kb/test-pilot-test-firefoxs-experimental-features SUMO community discussions; Firefox Test Pilot https://support.mozilla.org/en-US/forums/contributors/711977

Test Pilot is controlled from this page: https://testpilot.firefox.com/experiments/

From there you can disable the experiments one-by-one and also provide feedback about each feature that you disable.

Use the Settings button to access the menu for support and information about Test Pilot. https://discourse.mozilla-community.org/c/test-pilot

I have reviewed the Experiments currently active from my Test Pilot page and have temporarily disabled Page Shot and Min Vid Experiments. I gave feedback that it - in a way - 'breaks' Firefox and so far, the browser performance has improved.

I'll continue to check on it and will see which other Experiments to close in case Firefox performance is encountering a sluggish issue again.

That was very good work. Well done. Please flag your last post as Solved Problem so others will know.