You're not the only one. I have been trying to tell them for weeks the memory use is probably the reason people are saying it's slow. It's not unusual for the beta to blow up to 600mb or more, and if someone has an XP system with 'only' a gig of ram, it would kill the machine. MY thing is beta 12 (coming shortly) was supposed to be the Release Candidate. And they seem to be making no effort at all to address the memory issue. I have been using Firefox since like 2006, but I'll stop using it if they don't fix this.

Having said that, I hope they do. I really like FF 4.

A high memory usage in Firefox does not mean necessarily that Firefox is the culprit. Extensions and plugins are other possible culprits.

See the Firefox uses too much memory or CPU resources - How to fix article.

If after further investigations, you confirm it is a leak issue, please file a bug.

Alright. I've disabled all add-ons and themes (and restarted, obviously), and have confirmed that there is some sort of leak that causes Firefox not only to steadily increase in memory usage, but also refuse to free up memory after closing tabs, even when it's not doing anything. I'm already filing a bug report of this.

Definitely a Firefox 4b11 issue. It's slowly using up all available RAM and doesn't release it unless closed. After about an hour of browsing I had 1.6GB of RAM usage on my MacBook Pro 13" with 6 tabs open. That was with all Add-On's disabled. I'm glad they fixed the CPU Usage problems with b11 but now there's a serious Memory Usage issue. I hope they can fix this, it's unusable after an hour of surfing.

Still an issue with FF beta 12. My RAM usage was spiking between 1.6gb and 5+gb (out of a total 6gb) when viewing certain pages. The worst culprit happened to be Sony PlayStation Network page. It rendered my x58 system almost unusable it was that laggy and slow. I've confirmed the pages run fine with IE. Task manager processes showed Firefox using approx 4gb of resources. It definitely looks to be either an extension or plugin issue as it only happens on certain web pages. Typing this text right now with FF beta 12 no issues at all.

I've found that this happens to me as well, I looked into it a little bit and found that it (seems) to be that when you open up flash content firefox allocates all of the ram space needed for the flash content to run, but when you close the flash content firefox isn't "notified" that the ram space previously needed is now no longer needed, I tried installing a flash blocker which (seems) to have worked reasonably well (I closed a tab when I started testing and lost 30Mb ram usage, even though it shot back up soon after) , though even now with firefox only been going for around 10 minutes so far and this being the only tab I have open or have opened I'm still sitting at 177 Mb (and is steadily increasing by 2-5Mb per minute), I mean I 'only' have 1 Gb of ram space, I only have 2 extensions installed and the basic theme with 3 plugins so that is quite alot.

Firefox 7 uses about 30% less memory than Firefox 6. See: http://blog.mozilla.com/nnethercote/2011/08/09/firefox-7-is-lean-and-fast-2/

I got talked into Firefox 7. It was suppose to fix all the things the prior versions frustrated me with. Instead it is even worse. Grabs 350mb of memory almost as soon as I start it. Within minutes it is up to 475mb. A little more surfing and it inches closer and closer to a gig used and the system slows to a crawl. Earlier, I played an a youtube video embedded in a yahoo site and Firefox then grabbed ALL of the system memory. The issue with quitting and trying to restart Firefox and getting the message that it is already running is STILL present in version 7. A quick check of processes reveals that Firefox.exe indeed is still there and hanging onto about 466mb of RAM. I think Firefox has a lot of promise but is simply broken and has been for a long time.

Ok, I found the solution (for at least my boxes!) Yay!

The trouble is that Firefox changed the amount of HTTP connections it permits simultaneously from 30 (pre version 6) up to 256 for version 7. Each connection uses CPU and Memory!!!!!

I won't ponder why they did that, but the fix is easy:

Lower the max number of HTTP connections

  In the Location bar, type about:config and press Enter.
      The about:config "This might void your warranty!" warning page may appear. Click I'll be careful, I promise!, to continue to the about:config page. 
  In the Filter box at the top, type network.http.max-connections.
  Double-click on the network.http.max-connections preference, the "Enter integer value" dialog box will open.
  In the field, type 30 which was the default value in old versions of Firefox and click OK.

My version 7.0.1 firefox suddenly started working! Yay!

gmiddleton, That did not work for me. I am also on 7.0.1 but no "Yay!" for me.

You can limit the amount of RAM Firefox is allowed to use. The following worked for me:

1. In the Location bar, type about:config and press Enter. The about:config "This might void your warranty!" warning page may appear. Click "I'll be careful, I promise!"

2. In the Filter Bar type: browser.cache

3. Select: browser.cache.disk.capacity

4. Double click on: browser.cache.disk.capacity

   You will then be able to change the capacity value. 

5. After changing it click on: OK.

Mine was set to more than 1,450,000. It should normally be set to a default of something around 50000. The good news is that you can lower it. I changed mine t0 100,000 and it helped A LOT!

If you have less than 1GB of RAM try a lower number, something between 20,000 and 15000.

Good luck!

http://maketecheasier.com/28-coolest-firefox-aboutconfig-tricks/2008/08/21