I Have exactly the same problem. FF won't stay below 50 per cent CPU.

I also have this problem which has been getting worse with recent successive updates of Firefox. As a long-time user and advocate, I am about ready to look for an alternative browser. Firefox will not close and exit properly, requiring direct cancellation through Task Manager; starts up very slowly (often 1 to 2 minutes); and frequently locks up the CPU with 100% of cycles while doing NOTHING; offers to restart crashed sessions, but cannot recover and reload those sessions; will not operate correctly while any other application accesses the internet, like uTorrent. Firefox is set to delete cookies, etc. whenever the session closes, sqlite database is vacuumed regularly, CCleaner regularly wipes remaining accretions, and disk is defragged. Currently running 3.5.5 but will NOT upgrade until this problem is solved since later builds report the same problems. I know several other users who have experienced the same problem and I can no longer automatically recommend Firefox to users and clients. Is anybody paying any attention to this problem? What is being done to address this fault? Why is there no response to the original poster? Is this really a support forum or just a way to preoccupy and distract the users who suffer this malfunction? Wireless 54Mbps connection. No Adobe Flash, QuickTime, MS Silverlight, etc. installed on this machine. Firefox 3.5.5 with the following add-ons:

Aardvark 3.0 Adblock Plus 1.2.1 Adblock Plus: Element Hiding Helper 1.0.6 Free Download Manager plugin 1.3.4 Highlighter 0.1.4 MR Tech Toolkit 6.0.4 repagination 2006.4.5.1 Save Complete 1.0.1 Session Manager 0.6.8.3 Tab Mix Plus 0.3.8.4 Unhide Passwords 1.2.5.1 User Agent Switcher 0.7.2

Thanks for any help or suggestions.

See http://kb.mozillazine.org/Firefox_hangs and Firefox hangs or is not responding - How to fix (Hang at exit)

I have FF 3.6.8 and been experiencing heavy cpu usage. In the previous versions I would have the same number of tabs open (15-20) and wouldn't have any issues on my core 2 duo box. There have been no additional plug-ins added. And the more interactive the website the slower it gets. I would constantly get pauses in ff which I would have to wait for it to unlock.

So the only fix that I can think of is uninstalling the whole program and that means everything. Lose everything that is apart of ff - addons, history, etc. and reinstall. Good luck.

I have Windows XP. I deleted the prefetch file for Firefox located in (C:\Windows\Prefetch\FIREFOX*).

Doing this stopped the 100% cpu usage problem.

Deleting prefetch file doesn't help - still sucking up CPU time after deleteing the file - plus Firefox recreates it when restarted. (Using 3.6.11)

This problem as been around for some time/thru many versions - at least year.

I'm experiencing similar problems:

1) I'm using FireFox v3.6.12 2) When running FF my system gets very sluggish, particularly other apps such as MS Excel. 3) Task Manager shows memory useage to increment regularly and is not released until FF is exited. 4) Task manager shows CPU useage jumps to close to 100% on "exiting" FF in the normal way, but still shows FF to be running. 5) It's necessary to close FF in Task Manager in order to shut down Windows normally, otherwise nsAppShell:EventWindow shows up as locked and needs to be shut down.

Hope this helps.

I have the same problems as all above. I have windows xp pro and the cpu hits 100% and freezes everything. I checked my "add ons" and updated all I could [especially the flashdrive or whatever] and now it still maxes out at 100% cpu usage but does not freeze as frequently. IE is also 100% my cpu , especially with idiot ads as others state above.

Anyone gets a solution post it.

I have the same problem when I close firefox my cpu usage pegs at 24 to 28 percent until I kill the process but I also have a similar problem with a process called under description as windows media center and I am not sure what triggers this as I never open media center or media player. any assistance would be helpful.

I have had the same problem with Windows xp sp3, and to a lesser extent with vista 32 bit sp2, using several 3.x versions of Firefox. As elsewhere indicated with Google searches, a possible solution appears to be the addon Memory Fox by IDEVFH . Even though initially it is reported as incompatible with Firefox 3.6.13, I have found it to be a perfect solution in terms of controlling CPU excess usage. I wish you all a happy and productive New Year.

Hope we see a solution to CPU problem soon. I am a big fan of Firefox but about to leave it because of this problem..I will try Memory Fox by IDEVFH before I leave though..

I have suddenly the cpu usage problem on Macosx 1.6 and Firefox 3.6.13. I checked my "add ons" and updated and i tried to switch them off but nothing.

Thanks folks you have answered my question, where Firefox central support did not.

I originally asked which version of Firefox is the last stable version for older PCs as I was considering updating. Now I will NOT!! I was having CPU pressure problems with upgrades a way last year and did some research back then to find out that the last stable, non-crash version for older machines was and apparently still is - Firefox Version 3.0.19 - believe it or not.

However, with this version Firefox is at pains to broadcast that this version is no longer "safe". But if Firefox is only going to write software for i-5 dual cores and above computers then those with average and way below average machines are going to have to use OLDER versions of Firefox along with other less RAM hungry 'protection' software.

Currently My line-up is :

Firefox Version 3.0.19 (Old but NO bother, like a charm and add-ons :)

PC Tools Anti-virus (free & not the Spyware doc version) THE lowest RAM on the net (believe me I have tested them ALL!)

Advance System Care Professional (on) and NO trouble. The PRO is what you want.

IOBit Security 360 (on) and NO trouble (Free version is just fine, although I have Pro).

And if you are really F***ed - RAM Booster Free II (Again a sweet one, no trouble at all.)

And my system - a Dell Pentium 4

Now you have an answer. The very BEST of luck!

Crikey