I have long been a proponent of the Firefox browser and a regular user of it since well before the official first release. This despite the few niggly little bugs that existed in the early days, mainly memory leaks and problems with Javascripts (actually I think some of those still exist). Even the need to regularly shutdown and restart Firefox to free up memory didn't deter me from using it as my preferred browser.

However the recent upgrade to version 2.0.0.1 seemed to introduce a new issue that proved to be a real show stopper as far a Firefox was concerned. I found that after Firefox had been open for a while it would start consuming every available CPU cycle on the machine (which, by the way, has 512Mb RAM and is running Windows Home Edition with SP2) and effectively ground the PC to a halt. So much of an issue was this that I had to switch back to IE7 while I tried to resolve the problem.

First step was to check the Mozilla Support Forum to see if this was an isolated event or was anyone else was experiencing this issue. There a found a lengthy thread under "2.0.0.1 performance problem" that indicated I was not the only user experiencing these problems. The thread also suggested an number of varied possible causes for the problem including:

The Flash plugin (this from Robert Lindsay's blog "Fix For a Nasty Firefox CPU Bug ")
A variety of Firefox extensions
Using WYSIWYG rich text editors
Conflicting Firefox profiles

So that gave me a number of possible root causes for my CPU usage problem. Well I typically don't visit sites with Flash so I thought it was unlikely to be the Flash plugin that was causing the issue. There is a Flashblock add-on that can be used to block Flash (you'd have never have guessed that would you); but as extensions are a possible source of the problem, and I didn't think Flash is an issue; I didn't want to install yet another possible culprit.

Next comes this issue of the extensions I'd installed. Over time I'd installed quite a few extensions to try them out; but the only ones I used with any regularity were Forecast Fox and Web Developer. Unfortunately Forecast Fox is one of the extension mentioned on the "Problem Extension" page as having memory leaks (though no mention of CPU gobbling) so I thought it best to uninstall this and all the other extensions to see if that would cure the problem.

Using a WYSIWYG rich text editor is something I do all the time for maintaining this and other web sites and so abandoning this form of input was not really going to be an option. These editors abound on blog sites too to I was really hoping that this was not going to be the problem. I decided to leave this possible cause to the end and only address it if nothing else worked (actually I would probably switch to another browser than abandon using rich text editors).

The final possible cause was conflicting profiles. Now I've never used anything other than the default profile; but I did read elsewhere that installing version after version on top of each other can lead to problems in the profile. So this was a possibility that needed to be addressed.

In the end my plan of action was not very scientific. Rather than make just one change at a time and observe the effects I took an all or nothing approach. I uninstalled all my extensions and then restarted Firefox using the profile manager (run firefox.exe -ProfileManager). I created a new profile and deleted the default profile. I think this latter action would have got rid of my extensions too; but there you go.

So on restarting Firefox with a new profile I was back with a default browser; no extensions, no themes and no excessive CPU usage either. In fact I've had 3 tabs open for about 3 hours now and Firefox is showing 0% CPU usage, so I've done something right. I've now installed a couple of themes too and that doesn't seem to have had any effect. Instead of installing Forecast Fox I've installed the OneClick Weather extension which reportedly doesn't suffer from memory leaks. However it's a pretty inferior product, in my opinion, with a much reduced set of icons and the ability to forecast either day or night conditions; but not both. So I'm going to retry Forecast Fox again in a minute to see if that will run without any CPU issues.

The other thing I haven't tried yet is using a rich text editor (I'm typing this into a plain text editor (Textpad )) while I observe what Firefox does. So I'll try the WYSIWYG editor shortly and see if that has any effect on CPU usage. Visiting a site with Flash (www.adobe.com ) doesn't seem to have made an appreciable difference to the CPU usage except for an occasional 2% blip that wasn't there before and closing that tab seems to have reverted the browser to its previous CPU usage of 0%.

So that leaves two things to try; re-installing Forecast Fox and using a rich text editor. I'll report back later if either of these cause the CPU usage to increase again.