Further to ronanpaizao post, I wonder if this has anything to do with it? Flaw in WebGL see The Register for details-:

http://www.theregister.co.uk/2011/05/11/chrome_firefox_security_threat/

The advice being given is to turn WebGL off in 'about:config'. Note WebGL applies to latest versions of Chrome, Safari and Opera as well as FF4 & 5.

Thought I'd pass it on before you are hacked! Reading between the lines WebGL don't care two hoots about the flaw!

Microsoft won't even touch the software but that's another story, we can guess why though, they like to keep 'their hands on the pot'.

WebGL is the 3D rendering engine, here is a link on how to turn it off in Firefox and Chrome-:

http://techdows.com/2010/12/disable-webgl-in-firefox-4.html

a strong supporter of Mozilla this has been a step too far. Today I have not had to reboot my computer once. Browsing and other programs have worked fine without grinding to a halt. What have I done .. defaulted my browser from Firefox 5 to Opera . I am so happy as I have had so many issues last week a full reload was on the cards. Trouble is if the problem is cured do i really want to come back ?

Even at firefox6 beta the hangs still occur frequently. does anyone have a solution? It's really annoying

Agree with all of you. It appears that FF5 is having some major issues when it comes to flash video playback. I get jittery video, unresponsive windows, and major load times for webpages @ 10secs +. I'm a technology specialist for one of the worlds biggest IT companies (Not MS) and i ensure that i have sufficient hardware and that other resources aren't "hogging" the memory that the system is clean and patched and still have these issues. I have to say that i will stay with FF until there next patch, however, should the issue persist i recommend you going across to Chrome (bypass IE) or if you really are a hardcore FF fan then roll back to 4 until they sort this out. What we all must remember though, THIS IS A FREE PIECE OF SOFTWARE, they dont owe us anything, we dont give them anything so cut out the bad language and the aggressiveness' and grow up. I say to the DEV's i appreciate all your hardwork, it does however seem that FF5 is a little buggy and we would appreciate it if you could address these problems.

Thanks Again

Poomonkey :P

Now't is free for now't.... nothing! Not even Firefox or any other, there is always some trade off Remember Phorm - read this http://www.theregister.co.uk/2008/05/19/firefox_data_snoop/ and a follow up from the horses mouth, http://john.jubjubs.net/2008/05/13/mozilla-firefox-data/ sure makes me wonder if Firefox is really free!

This ain't the problem here, the usability has gone from crashes to mediocre! They certainly can't be ahead if they produce stuff that causes problem after problems.... about time these guys (inc MSFT) catered for the user with an average computer.

The software ain't all that well made or secure if they keep adding updates after updates 'security fixes' after security fixes!

There is a 5.01 update on firefox.com that doesn't automatically download or appear when searching for updates. This provided immediate relief though it is still slower that Chrome.

i think i found the Solution ,i run this scan: http://www.eset.com/us/online-scanner and its find injectvirus at firefox folder , and now there are no more hangs , worth a shot!!!

I've been experiencing the same issue as described on a 64-bit install of Vista on a notebook and have found that clearing my cache (I manually set the cache size to 64MB, down from the Firefox managed size of 512MB), disabling hardware acceleration and the internal spellchecker have, for now, remedied my issues.

After having been a loyal and grateful Firefox user for years, I've completely stopped using your product. Since version 4 came out (I'm on version 5 now) this browser hangs, pegs my CPU usage at 100%, crashes, causes my entire system to hang, and sometimes requires a reboot. I have tried many possible solutions and the problem persists. I don't have time for this. My conversion to Chrome took 15 minutes and, so far, that product is working perfectly.

I have used Firefox as my primary browser on my MacBook Pro for years. It's always been my favorite. But since I updated, have had so much trouble with Firefox 5 freezing and crashing that I have given up. My new primary browser is Chrome, it's faster and best of all it doesn't freeze and crash.

Mozilla, if you want me back, better come up with something better than Firefox 5.

I had the same problem and I may have found an answer. I disabled the Integrated Gmail add-on and CPU usage in Task Manager for "firefox.exe *32" process went from about 22-48% down to 0-5% and memory usage is about half. Firefox seems to be running at normal speed for me now (so far).

Interesting - different people disable completely different add-ons and extensions and get better results - yet I don't have any of these add-ons or extensions, so they can't be the ultimate problem or my FF5 wouldn't have the problem. I tend to keep my add-ons to a minimum, and only keep the ones I use regularly, and the only thing that has helped is to completely shut it down on a regular basis (hourly works best) and restart after a few minutes. :-(

Thought I'd try downloading Speedyfox to see if that helps at all. FF needs to be closed to run Speedyfox so I closed it and still got the message tellling me that FF needs to be closed. Tried to cancel/close it from Task Manager and that didn't work. I have to reboot in order for FF to really close. As others have said I've been a FF user for years and this version has been bad news. When will we get a fix for this??? From the notes, you can see that this has been going on for at least a month now. I think it's time!!

I've been having this problem as well. All of a sudden, firefox will become unresponsive and I get the little hour glass. The CPU usage is at 50%.

So I decided to try it in firefox safemode since I've read it could be an add-on or extension. However, it just happened in safemode too. So it's not an add-on or extension. I've got a firewall and virus scans come back clean.

There is an issue with the browser and it started a while ago. I also have the problem where trying to clear cache seems to hang the browser as well.

I'm a long time user and don't really want to switch but this is not getting better. I was hoping to find it was an extension but it's not.

FIRE FOX EATS SHITTTTTTTTTT! I THOUGHT THERE WAS SOMETHING WRONG WITH MY PC ALL THIS TIME! But now, i should have known better, i have 10mb web speed! and my internet shouldnt be going slow like fire fox was.

The only plugins i had installed were, spell check + youtube to mp3 + smooth scroll.

But it shouldnt have crippled firefox like it did. Im on dual core 2gb ram pc!

I would be clicking web pages, and they would take quite long and lag, so I'd decide to go to the previous tab to carry on reading while the other tab loads, BUT OH WAIT, IT WAS GOING SO SLOW THAT IT FROZE FOR A FEW SECONDS WHEN I TRIED CHANGING TAB WHILE IT WAS LOADING.

BUT I TRIED GOOGLE CHROME, AND NO PROBLEMS WHAT SO EVER, WORKS PERFECTTTT. EVEN WHEN LOADING, ITS SO SMOOTH CAN MOOTH THE TABS AROUND IN AND OUT OF EACH OTHER WHEN EVER I PLEASE.

GET LOST FIREFOX, THIS IS 2011 AND YOUR BROWSER IS CRIPPLING PCS? ITS NOT EVEN A INTESNSE 3D GAME OR SOMETHING... JUST A BROWSER for viewing FORUMSSS

And of course, logging in and posting doesn't work properly, and has to be done again, because even after logging in I'm told I don't have permission to access this page.

Who the HELL do you think you are?

Mozilla have quite deliberately caused inconvenience to 100s of thousands of people, calculating, I suppose, that most of us will not go to the trouble of using a new browser. Well, i think this is is just so bad that will backfire. I really hope so. Your marketshare is falling and soon the hounds will catch the fox and tear it to shreds.

I wanted to post to say I've experienced just about all of the above, and have found a fresh disease - I can no longer use YouTube Clip Extractor (a standalone, notone of the plugins or add-ons you've messed up) to record from YouTube.

Thanks Firefox. Now please die.

I thought it was my laptop. It's an AMD Quad-Core N970 64-bit processor, 4gb RAM, 600GB HD and 128MB AMD ATI video card. I thought it was a driver problem. Or some incompatibility thing. It's not. It's Firefox. I'm writting this down on Chrome and my computer isn't hanging anymore. It runs like a charm and it isn't freezing. I'm kinda sad. Chrome doesn't have two of the features I liked the most from FF. The search bar and its many search engines and the Context Search add-on. I'm staying with Crappy Chrome until you guys fix this crap.

By the way. This is all kinds of wrong. 85mb RAM with two tabs? Compare that to Chrome. I've used FF since it was a Beta test, before 1.0. The first thing I downloaded on every new computer I set up. I hate to say this, but not even IE slowed down my computer this much. And don't get me started with the damn add-ons because even in safe-mode it slows the crap out of it.

The occasional unresponsiveness (10 seconds every 2-3 minutes) of Firefox 5 and 6 on 64-bit Windows 7 has caused me to discontinue it in favor of Google Chrome. Major problem for me, as a web developer, dependent on many of the AddOns for Firefox. I've tried disabling all AddOns and other, suggested tweaks and Firefox is has morphed into crap on the 64-bit platform. For unknown reasons, Firefox 5 and 6 work pretty well on my netbook's Windows XP platform but Firefox is unusable on my main desktop PC.

Ducky! Just ducky! I just downloaded the latest 'fix' and now the back button doesn't work! If I page forward in an article, or follow a link, the back button remains grey as it I just opened a new tab. This sucks!