What version of Firefox? Have you tried Windows in Safe Mode? Turn on the computer and immediately begin pressing the F8 key repeatedly. From the Windows Advanced Options Menu, use the arrow keys to select Safe Mode, and press ENTER. The computer will automatically exit Safe Mode when it is shut down.

Start Firefox using Troubleshoot(Safe) Mode {web link}

A small dialog should appear. Click Start In Troubleshoot(Safe) Mode (not Refresh). Is the problem still there?

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

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-uses-too-much-memory-ram

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

FredMcD,

Starting in Troubleshoot mode cures it but how do I find the issue? I only have one add on which is AdBlock Plus.

Jonzn,

Also starting Windows in Safe Mode cured the lag, but again here, how do I find the issue?

Firefox is 99.0.1

What security software are you running? Disable it and see if it makes a difference.

Running Avast AV only. Turned it off and there is no difference. I also tried Firefox in Troubleshoot safe mode that started immediately. I turned off the Ad Blocker and retried FF in normal starting mode and the startup lag persisted.

I saw the written suggestion to change the background theme to default but I never changed it from the original install and none of the backgrounds were called default. It is running "System Theme - Auto".

I have used Firefox for a long time over Chrome, but now it's gotten to be not worth the trouble because Chrome works without a hitch and this seems to be a dead end.

Running Avast AV only. Turned it off and there is no difference. I also tried Firefox in Troubleshoot safe mode that started immediately. I turned off the Ad Blocker and retried FF in normal starting mode and the startup lag persisted. I saw the written suggestion to change the background theme to default but I never changed it from the original install and none of the backgrounds were called default. It is running "System Theme - Auto". I also did a fresh install prior to posting to the forum and that did not solve it. I might guess that this happened after the last update but I am not quite sure about the timing. I have used Firefox for a long time over Chrome, but now it's gotten to be not worth the trouble because Chrome works without a hitch and this seems to be a dead end.

Uninstall Avast and test. I'm in Win7 and running ABP(see screenshot) with no Firefox start up issues. You already demonstrated by running Firefox and Windows in Safe Mode that the browser starts normally.

johzn, As I previously noted, turning off AV did not make a difference.

I had Avast and Firefox on for a long time and they worked fine together. Something happened recently that affected Firefox and all I and all I recall is Firefox update, but who knows.

It's just odd that starting in Troubleshoot start mode fixes it and turning off the one add on shown does not fix the issue. Something is not being reported in the Troubleshoot start mode that is apparently causing the problem when not using Troubleshoot mode.