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If one watches Firefox's CPU consumption, one can see it consuming large amounts of CPU even while it's not being interacted with. On lower-spec computers, this can cause performance problems that significantly affect other applications.

Is there any way to find out what Firefox is actually doing, so one can try and change some configuration options to reduce its CPU consumption?

It's common knowledge that addons can affect performance, but the install/uninstall dance isn't exactly efficient. Does anyone have a better method for finding something that could point towards what might be causing these issues without having to search behind the couch?

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I think there might be some good suggestions for hunting Firefox performance issues, so I made it community wiki. – wfaulk Oct 15 '09 at 14:54
Possible Duplicate:… – EvilChookie Oct 15 '09 at 15:29
up vote 4 down vote accepted

After opening Firefox, it might be updating bookmarked RSS feeds. Using, for example, the Live HTTP Headers add-on, you can see that and any other traffic, even though it might seem nothing is being downloaded.

(And as a side note: if it's slow then vacuuming the database that holds the bookmarks and visited sites might help.)

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It hadn't occurred to me that there might be things downloaded in the background. Good idea about Live HTTP Headers. – wfaulk Oct 15 '09 at 14:22
@wfaulk: this can in fact be quite annoying if you want to debug something but did not yet run Firefox today ;-) (And, Firefox may be looking for software updates as well, and may be fetching a new list of scam domains.) – Arjan Oct 15 '09 at 14:31
I'd forgotten a bunch of "Live Bookmarks" deep in my bookmarks structure. Getting rid of those helped my immediate issue out a lot. – wfaulk Oct 15 '09 at 14:53

This is a bug in Flash.

I have no idea what has caused it (and written about it in previous answers). It is something about certain movies that when compiled, take up very high (sometimes maximum) amounts of CPU when in any web browser.

I have a few complete flash sites where the CPU never goes above 2% and then I can see a tiny advert in a square on a site and the CPU goes to 100%, so it must be a bug.

I advise you install Adblock plus as I find it very good and has nearly completely stopped this problem. If you want to get rid of all Flash, you can try Flashblock.

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i <3 Flashblock :) – quack quixote Oct 15 '09 at 15:15
NoScript will block Flash, too. – wfaulk Oct 16 '09 at 18:03
@wfaulk: but Flashblock gives me a nice shiny button to press if i actually want to see the flash... – quack quixote Oct 19 '09 at 13:40

Look at your pages. Flash and animated images seem to be driving my Firefox instances. A lot of pages that used to be static seem to be adding Flash.

The other indicator of CPU usage I look for is the speed the fans are running.

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Not only flash but look at any potential add-ons you may have. These are not written by Firefox/Mozilla and are sometimes the source of such problems.

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Great addon for reducing memory usage/leaks: AFOM

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