on winXP / SP3, issue is seen most on videos

Too many plugins can cause firefox to slow down. Try removing some.

Using Vista, ever since upgrading to Firefox 3.6.6 my browser is extremely slow. I haven't added any plug ins recently ... the only change is this upgrade.

How do I go back to the old version? Or perhaps I should go to another browser.

I have a mac, and the update completed automatically... and everything is so slow. Has this update worked its way into the operating system?

I have never had to remove plugins in the past. How do I do this?

Under Preferences > Applications - all I can see is "content type" and "action". Under "action" it shows "use" these three, which I can not remove because I need them:

Quicktime 7.6.4, Shockwave Flash, and RealPlayer.

Are other plugins somewhere in my computer?

It's exactly the same for me. The only add-on I have installed myself is AdBlock Plus and I've never had problems with it. It often takes around a full minute to load a webpage, which is the most annoying thing ever. It started since the 3.6.4 update and the 3.6.6 update didn't fix the problem..

Same problem, on Vista.

not a plug-in problem.

Same problem of slow web page load on Win 7. No change in Firefox or plug-ins other than the update.

windows xp, lots of tabs: FF has become very sloppy, and cpu intensive; my laptop's fan is working very hard

Same problem on windows 7, just changed to older version and it is fast now.

Apparently I'm not the only one with this problem, I run on XP

Seeing this as well, after update from 3.6.4 to 3.6.6 Firefox is using excessive CPU (25% on a Quad-Core 3.2GHz), and flash video is very jittery even in a small window. Nothing else changed since update. Possibly of note is that the plugin-container is only using 3% CPU when it usually uses much more while playing streaming flash video (justin.tv in this case).

Possibly unrelated but another issue that cropped up is that I can longer drag the main window itself around either... as soon as I start to, it locks up the window until I Alt+Tab to another window, and then back, and when going back, Firefox jumps back to the position it was in before I started dragging it.

Going back to 3.6.4 now.

Well, this may not help anyone else, but I figured out my issue.

After reinstalling 3.6.4, it too was doing the same thing, using 25% CPU consistently, unable to move the window around, etc.

As bizarre as it sounds, this seems to have been an issue with having a tab open playing flash video during an update/install.

After 3.6.4 was doing it as well, I closed the justin.tv tab and restarted 3.6.4, opened justin.tv back up and it was fine using 2-4% CPU or so instead of 25%. Window can be moved around again too.

Downloaded the full 3.6.6 installer, closed the jtv tab, reinstalled 3.6.6, launched, and reopened a tab for jtv... fine still using 2-4% CPU tops.

Very bizarre. Nothing else changed during any of this other than whether I had JTV open and playing video during the update/install and subsequently if it would be loading as soon as it restarted after the update/install.

After the initial startup, I can close and reopen Firefox as much as I want with the JTV tab open and playing and it has no further issues.

Weird.

Hopefully someone on the support team notices this problem and can provide an update soon. FF is all but unusable on my top of the line desktop after the 3.6.6 update.

Does anyone have a link to the 3.6.4 build so I can reverse to an older build?

In case anyone is having performance issues and would like to back-date to an older version of FF, see here:

http://www.mozilla.com/en-US/firefox/all-beta.html

This is the link to the most recent beta releases. It currently is hosting 3.6.4 beta7.

I installed it and boom... performance was as fast as it had been.

PS. Good job on the improvements with 3.6.6 Firefox dev team, although this one catastrophic problem seems to be impacting a group of users.

After back dating to the 3.6.4 beta I downloaded the full installer for 3.6.6 .

Per Melir's instructions, I then closed any active instances of firefox and installed 3.6.6 fresh from the downloaded installer.

Works fine now.

the auto updater must have a bug that only occurs when the program autoupdates or attempts to update when the client is still running.

Good work Medlir!!!

Windows XP Firefox Plus all applications including MS Word slowed down

Tried getting rid of extensions, disabled plug ins, most of which I had not installed. FF does this automatically?

Just to add my voice to the group, since the update to FF 3.6.4 (and then 3.6.6) any youtube video at standard res is playing choppy - this as a reference, before they just run "fine".

The bottom line is that *any* flash video I play is slow on framerate.

What's happened to FF? it's not a config issue, I've same setup on a netbook (so, low power) and it became sluggish too :( - IE plays fine instead :\ (or Chrome).

Yes, I've update every plugin, and disabled any active add-on that might impair playback, however CPU utilization's not right, too high, and also idling FF with 8-10 tabs opened the CPU is spiking up/down pretty fast.

I use winXP-sp3 (1Gb ram), and the windows task manager is reporting a grand total of 21 processes including the 4 AVG ones (so, exxtra trimmed down, no bugware or whatsoever).

osx 10.6.3, ff 3.6.6 very slow, 3.6.4 was ok !

same problem slow pages sometimes even times out. Only change was the automatic update to 3.6.6