I notice that you do have a lot of extensions installed including Firebug.

Did you try a new profile without any extensions?

See "Creating a profile":

• https://support.mozilla.org/kb/profile-manager-create-and-remove-firefox-profiles
• http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Profile_issues

If the new profile works then you can transfer some files from an existing profile to the new profile, but be careful not to copy corrupted files.

• http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox

No, but I don't see how that would help. That doesn't really address the issue of why RAM usage has ballooned three-fold between Firefox 3.6 and Firefox 21, unless I'm missing something (which is possible)...

As I said, Firefox 3.6 is using 1/3 of the RAM with the same extensions installed (the troubleshooting log I pasted in was from the 3.6 session).

charles, please update to Firefox 21, then try resetting Firefox Refresh Firefox - reset add-ons and settings. Using an ancient profile from Firefox 3.6 can sometimes cause weird issues in newer versions of Firefox, so a reset is needed. Plus, you don't want to run Firefox 3.6, it's terribly insecure and not supported anymore.

I was running Firefox 21, that's the problem. For example, how much RAM is you Firefox 21 using at the moment? I'm guessing 700MB+. My Windows 7 installation of 21 uses less than my Mac OS 10.7 installation of 21, but it's still using ~800MB. Does that not seem like a lot?

Firefox 3.6 *does not* have the problem under the exact same circumstances. With the same tabs, bookmarks, addons, etc. Firefox 3.6 is using 300MB of RAM.

I don't want to run 3.6, you're right, but if it has 33% of the memory footprint, that's gonna be a clincher, isn't it?

My Firefox 21 uses around 250mb of RAM, but you can't compare computer to computer.

Did you try updating to Firefox 21 and using Reset like I said?

Reset my Firefox 21 install, that did indeed bring the RAM usage down to about 300MB, however, that's with no add-ons installed.

With a fairly minimal set of add-ons, it's ballooned back up to 600MB.

So, a reset Firefox 21 with fewer add-ons is still using twice the RAM of Firefox 3.6...

Add-ons installed: Adblock Plus Download Statusbar feedly Hola Stylish

Maybe it has to do with the amount of tabs you have open? How many tabs do you have open usually? I find people using 10+ tabs are doomed to have high memory usage.

And more importantly, the content inside those tabs can really affect memory usage. Sites with lots of big images, animated gifs (like Tumblr), Facebook (b/c of all the Javascript it uses) and sites that use flash plugins like Youtube can cause high memory, in my opinion.

There's a lot of sites that just seem like huge memory eaters. If you can post some of the sites you visit, some of us can try to surf with those for awhile and see if we can get the same memory usage as you.

I believe the content on the page + a high number of tabs open cause high memory usage. I personally try to surf with only 5 tabs or less. Using the back button to go to the 1st page in the tab then visiting another page or about:blank to erase all the history for that tab. I find doing that releases memory much faster.

Hmmm, I don't find that number of tabs has much effect. For example, at the moment, I have gmail, facebook, and google reader open. 600MB.

Searched a random word on Google, opened the first ten links (a mix of wikipedia, imdb, apple trailers, youtube, etc). 660MB. So quadrupling the number of tabs led to a 10% increase in memory usage... Which is fine. 6MB, on average, for a tab seems pretty reasonable to me.

It's the baseline usage that's high.

Hello,

Try disabling graphics hardware acceleration. Since this feature was added to Firefox, it has gradually improved, but there still are a few glitches.

You might need to restart Firefox in order for this to take effect, so save all work first (e.g., mail you are composing, online documents you're editing, etc.).

Then perform these steps:

• Click the orange Firefox button at the top left, then select the "Options" button, or, if there is no Firefox button at the top, go to Tools > Options.
• In the Firefox options window click the Advanced tab, then select "General".
• In the settings list, you should find the Use hardware acceleration when available checkbox. Uncheck this checkbox.
• Now, restart Firefox and see if the problems persist.

Additionally, please check for updates for your graphics driver by following the steps mentioned in the following Knowledge base articles:

Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems

Upgrade your graphics drivers to use hardware acceleration and WebGL

Did this fix your problems? Please report back to us!

Thank you.

Tried disabling hardware acceleration, no change.

Previously tried a reset and gradual re-install of plugins. I don't use any themes.

Problem is primarily on Mac OS X (have the problem on Windows, too, but my Windows machine has enough RAM that it doesn't really matter). Mac OS X is up-to-date 10.7 with latest everything.

So, after a few days of using Firefox 21 again after the reset, memory usage has gone back to ~1GB.

Memory usage in Firefox 3.6 is still steady at about 300MB.

I'd really appreciate some help or feedback about why 3.6 uses so much less RAM than 21, despite the fact that Mozilla claims to have made improvements to memory usage in each version.

21 is almost unusable on my machine, but 3.6 works like a charm.

Is this with extensions or without extensions?

If you have installed extensions, can you tell us which?

Same extensions on 3.6 as 21, as I said above.

The extensions are (again, as per above):

Adblock Plus 

Download Statusbar

feedly

Hola

Stylish

Sorry, my mistake, Hola is not actually installed on 3.6.

But that's not the issue, as I had this problem with 21 before I installed Hola (which was fairly recently).

Did you try to run a profile without any extensions (so do not install Adblock Plus and Download Statusbar) for awhile?

See:

• http://kb.mozillazine.org/Creating_a_new_Firefox_profile_on_Windows
• http://kb.mozillazine.org/Shortcut_to_a_specific_profile
• http://kb.mozillazine.org/Using_multiple_profiles_-_Firefox

Use the -no-remote command line switch to open another Firefox instance with its own profile and to run different Firefox instances simultaneously.

• http://kb.mozillazine.org/Opening_a_new_instance_of_Firefox_with_another_profile

• https://developer.mozilla.org/en/Mozilla/Multiple_Firefox_Profiles

Similar to resetting Firefox, using a new profile does indeed fix the problem.

RAM usage is about 360MB.

However, this is without extensions. Reinstalling the extensions will undoubtedly reintroduce the problem, as it did with resetting Firefox.

Why would the same set of add-ons work fine in 3.6, and cause enormous memory usage in 21?

If extensions aren't working properly and leak memory or otherwise use an excessive amount of memory the you need to contact the developer of the problematic extension and refrain from using it for now.

Having large exception list in Adblock Plus could be causing this and other extensions may store data as well.

Yes, but it's specifically a Firefox 21 issue - with the same extensions, Firefox 3.6 works just fine...

After you created a new profile, did you ever try installing your extensions into that new profile?