I just had this happen to me in FF3.5.3.  It happened just after I installed the ALEXA Sparky extension.  I removed it and everything is back to normal.

Dave

Generated: Thu Oct 01 2009 13:40:49 GMT-0500 (Central Daylight Time)
User Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9.1.3) Gecko/20090824 Firefox/3.5.3 (.NET CLR 3.5.30729)
Build ID: 20090824101458

Enabled Extensions: [34]
- Adblock Plus 1.1.1: http://adblockplus.org/
- Chrome View 0.2.0: http://www.systemantics.net/chrome_view
- Chromifox Companion 0.5: http://www.google.com/search?q=Firefox%20Chromifox%20Companion
- Chromin Frame 1.0.9.3: https://addons.mozilla.org/firefox/addon/10091/
- Coral IE Tab 1.60.20090901: http://coralietab.mozdev.org
- DOM Inspector 2.0.3: http://www.mozilla.org/projects/inspector/
- DownThemAll! 1.1.6: http://downthemall.net/
- FaviconizeTab 0.9.8.2: http://espion.just-size.jp/archives/06/308085916.html
- Firebug 1.4.3: http://www.getfirebug.com/
- Firefox Showcase 0.9.5.1: http://showcase.uworks.net/
- Google Pagerank Status 0.9.9: http://pizzaseo.com/firefox-pagerank
- Google Redesigned 0.3.5: http://www.globexdesigns.com/
- Greasemonkey 0.8.20090920.2: http://www.greasespot.net/
- Hide Caption 0.7.6.1: http://www.google.com/search?q=Firefox%20Hide%20Caption
- Html Validator 0.8.5.8: http://users.skynet.be/mgueury/mozilla/
- IE View Lite 1.3.5: http://www.graysonmixon.com/extension/
- Java Console 6.0.01: http://www.google.com/search?q=Firefox%20Java%20Console
- Java Console 6.0.02: http://www.google.com/search?q=Firefox%20Java%20Console
- Java Console 6.0.03: http://www.google.com/search?q=Firefox%20Java%20Console
- Java Console 6.0.07: http://www.google.com/search?q=Firefox%20Java%20Console
- Java Console 6.0.11: http://www.google.com/search?q=Firefox%20Java%20Console
- Java Console 6.0.13: http://www.google.com/search?q=Firefox%20Java%20Console
- Java Quick Starter 1.0: http://www.google.com/search?q=Firefox%20Java%20Quick%20Starter
- JavaScript Debugger 0.9.87.4: http://www.hacksrus.com/~ginda/venkman/
- Microsoft .NET Framework Assistant 1.1: http://www.windowsclient.net/
- MR Tech Toolkit 6.0.3.4: http://www.mrtech.com/extensions/
- ShowIP 0.8.17: http://code.google.com/p/firefox-showip/
- Site Information Tool 1.2: http://www.wmtips.com/tools/info/add-on/firefox/
- SkipScreen 0.1.07282009_url_fix: http://skipscreen.com/
- Sticky Password Autofill Engine 2.0.37: http://www.stickypassword.com
- Tab Mix Plus 0.3.8.1: http://tmp.garyr.net
- Web Developer 1.1.8: http://chrispederick.com/work/web-developer/
- WOT 20090918: http://www.mywot.com/
- Xmarks 3.3.2: http://www.xmarks.com/

Installed Themes: [7]
- Chromifox Basic 1.1.3: http://tcmediatech.com/firefox/
- Chromifox Extreme 3.0.7: https://addons.mozilla.org/en-US/firefox/addon/9427/
- Chromifox Extreme Blade 1.0.2: https://addons.mozilla.org/en-US/firefox/addon/11413
- Chromifox Extreme Carbon 1.0.3: https://addons.mozilla.org/en-US/firefox/addon/10883
- Default: http://www.mozilla.org/
- Noia 2.0 (eXtreme) 3.69: http://kasteo.deviantart.com
- Vista-aero 3.0.0.62: http://www.google.com/search?q=Firefox%20Vista-aero

Installed Plugins: (21)
- 2007 Microsoft Office system
- Adobe Acrobat
- Coupons Inc., Coupon Printer Manager 
- DivX Player Netscape Plugin
- DivX® Web Player
- Google Update
- IE Tab Plug-in
- Java(TM) Platform SE 6 U11
- Java(TM) Platform SE 6 U13
- Microsoft® DRM
- Mozilla Default Plug-in
- OpenOffice.org Plug-in
- Panda ActiveScan 2.0
- RealPlayer Version Plugin
- RealPlayer(tm) G2 LiveConnect-Enabled Plug-In (32-bit) 
- Shockwave Flash
- Shockwave for Director
- Windows Genuine Advantage
- Windows Media Player Plug-in Dynamic Link Library
- Windows Presentation Foundation
- Yahoo! activeX Plug-in Bridge