As a 60-second test (barely the time required to load two pages...), could you create a new Firefox profile to see whether this problem occurs with Firefox "uncustomized"? As you may know, in addition to bypassing current extensions, themes, and custom settings, a new profile should have completely fresh settings databases and a fresh cache folder.

Exit Firefox, then use Start > Run to run this:

firefox.exe -p

Click Create Profile and accept the default location (to avoid data loss, do not point the new profile to any existing location). Later, you can return to this dialog to start Firefox in your regular profile. (More info on the Profile Manager in this article: Profile Manager - Create, remove or switch Firefox profiles.)

Does Firefox work normally in the new profile?

• If not, program files may be corrupted, or there may be severe incompatibilities.
• If so, you may need to update extensions (or disable one or more of them) in your regular profile, or review custom settings.

Rather than type out 5 pages of possibly irrelevant material, could you let us know about the results of the new profile test?

please also make sure that your firewall isn't blocking any component of firefox or the plugin-conatiner & properly detecting the new version: Fix problems connecting to websites after updating Firefox

First, 90+% of the problem for me is around 3 tabs: Gmail Facebook Google searches (actually, loading clicked results; not search itself)

No, the problem is not remarkable in the new profile; but, there, I have only 3 tabs, instead of several dozen open tabs in this current session.

How can I go BACK to yesterday's version of Firefox?

Please, advise. Thank you.

This article has a link for old versions: Install an older version of Firefox. However, Mozilla already disclosed the security flaws in Firefox 20, so hopefully we can figure out your issue quickly so you can update again.

If you want to see how the new profile behaves with your full set of tabs from your last session in your old profile, you could try copying the sessionstore.js file from your old profile to your new one (when Firefox is closed), then relaunching the new profile. Due to the absence of saved data (e.g., passwords), it's possible that some tabs will not reload to the same state. To do that, use Help > Troubleshooting Information > "Show Folder" to open your new profile folder first, then exit Firefox, then navigate up a level in Windows Explorer to your old profile folder, open that, right-click, copy, and reverse direction back to your new profile folder, right-click, paste.

In your old profile, have you followed the typical guidance for website issues: clearing your Firefox cache and deleting your saved cookies for the sites at issue?

(1) Bypass Firefox's Cache

Use Ctrl+Shift+r to reload the page fresh from the server.

In this case, it would be more efficient to clear Firefox's cache completely using:

orange Firefox button (or Tools menu) > Options > Advanced

On the Network mini-tab > Cached Web Content : "Clear Now"

If you have a large hard drive, this might take a few minutes.

(2) Remove the site's cookies (save any pending work first). While viewing a page on the site:

• right-click and choose View Page Info > Security > "View Cookies"
• Alt+t (open the classic Tools menu) > Page Info > Security > "View Cookies"

This will open a dialog where you can remove the site's cookies individually.

Then try reloading the page. Any improvement?

Don't downgrade to an older version, instead try Resetting Firefox, Refresh Firefox - reset add-ons and settings

Did you try Safe Mode?

Start Firefox in Safe Mode to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance).

• Do NOT click the Reset button on the Safe Mode start window or otherwise make changes.

• https://support.mozilla.org/kb/Safe+Mode

Clear the cache and the cookies from sites that cause problems.

"Clear the Cache":

• Firefox/Tools > Options > Advanced > Network > Cached Web Content: "Clear Now"

"Remove Cookies" from sites causing problems:

• Firefox/Tools > Options > Privacy > Cookies: "Show Cookies"

Performance is OK in new profile including old sessionstore.js

Performance is OK in Safe Mode

However, Disabling ALL plugins, performance is still unbearably slow

What next?

I have identified the problem!

I disabled ALL extensions and plugins, then turned them on one at a time.

The culprit is ONE extension: Firebug 1.11.3

With all on except Firebug, my performance on this system is OK

In addition, when I enable Firebug, performance goes into the toilet

Disabling Firebug, and performance is OK again.

Interestingly enough, my home office XP PC finally updated to Firefox v21 yesterday. That, too, uses Firebug extensively.

I have had no adverse performance on that other XP PC

Please, advise.

Thank you

Great sleuthing! This has been reported to the Firebug developers: Issue 6458 - fbug - Firebug 1.11.3 makes webs incredibly slow on Firefox 21. I do not see any sign of a resolution there.

I have Firebug and haven't noticed a radical change. However, I only enable Firebug on a page-by-page as-needed basis, which is the default setting. To see whether you might be running Firebug unnecessarily, on the Firebug menu, turn off "On for All Web Pages" if it's on.

Perhaps my Firebug install was whacked?

On for All Web Pages was NOT visibly ticked

However, Tools | Web Developer | Firebug | Options | Reset All Firebug Options ... seems to have done the trick

I have restarted Firefox, including Firebug enabled, and my problem after Firefox upgrade is gone

Thank you - all of you!

jscher2000, hmm this is why exactly i switched to native devtools. resetting does makes things good. But jqery animations are still not smooth when i see them with firebug panel open.

they are smooth when i close the panel

UPDATE resetting firebug solves temporarily, but after few hours of working, its the same performance issue