Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of your add-ons is causing your problem (switch to the DEFAULT theme: Tools > Add-ons > Themes). See Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems and Troubleshoot issues with plugins like Flash or Java to fix common Firefox problems

If it does work in Safe-mode then disable all your extensions and then try to find which is causing it by enabling one at a time until the problem reappears. You can use "Disable all add-ons" on the Safe mode start window. You have to close and restart Firefox after each change via "File > Exit" (on Mac: "Firefox > Quit")

I have the same problem since a few days (WinXP, Fx 3.6.3). I'll try to investigate and let know if I found something...

I've checked the shared addons/plugins we have, and the most probably the error lies in Firebug 1.5.4 (or maybe, but less likely, in AdblockPlus 1.2 or the interaction between them).

Moreover, I've noticed the problem isn't only with closing two tabs instead of one, but generally duplicating some actions related with the tabs. I am using All-in-One Gestures, and the problems I have are:

• double closing the tab using middle click button in tab bar
• double closing the tab using AIOG "close tab" gesture
• moving two tabs left/right while using AIOG "next tab"/"previous tab" gesture.

After some time I realized that without Firebug the same happens. I found this topic where some people have the same problem: http://forums.mozillazine.org/viewtopic.php?f=9&t=1714135

So, well... my addons are: - Aardvark 3.0 - Adblock Plus 1.1.3 - Add N Edit Cookies 0.2.1.3 - Addon Developer Helper 0.05 - All-in-One Gestures 0.21.1 - All-in-One Sidebar 0.7.11 - BarTab 2.0 - bug489729 1.3 - Cookie Monster 1.0.0 - Custom Buttons 0.0.4.9 - Extension List Dumper 1.14.8 - FaviconizeTab 1.0.1 - Greasemonkey 0.8.20100408.6 - Image Zoom 0.4.4 - JSView 2.0.5 - keyconfig 20080929 - MD5 Reborned Hasher 0.7.3 - Menu Editor 1.2.6 - NoScript 1.9.9.87 - oldbar 1.2 - PermaTabs Mod 1.9.2 - Privacy Plus 1.0.1 - Right-Click-Link 1.1.5 - Session Manager 0.6.7.4 - Smart Middle Click 0.5.1.2 - Split Browser 0.6.2009110501 - Stylish 1.0.9 - Toolbar Buttons 0.6.0.8 - Undo Closed Tabs Button 3.6.2 - Web Developer 1.1.8

Plugins: - Adobe Acrobat 9.3.2.163 - Foxit Reader Plugin for Mozilla 1.0.0.1 - Java Deployment Toolkit 6.0.200.2 6.0.200.2 - Java(TM) Platform SE 6 U20 6.0.200.2 - LizardTech DjVu 6.1.1.1574 - Microsoft Office 2003 11.0.8161.0 - Microsoft® DRM 9.0.0.4503 - Microsoft® DRM 9.0.0.4503 - Microsoft® Windows Media Player Firefox Plugin 1.0.0.8 - Mozilla Default Plug-in 1.0.0.15 - Ogg Player Gecko Plugin 1.3.0.3 - QuickTime Plug-in 7.6.6 7.6.6.0 - RealPlayer Version Plugin 6.0.12.448 - RealPlayer(tm) G2 LiveConnect-Enabled Plug-In (32-bit) 6.0.12.448 - Shockwave Flash 10.1.53.64 - Silverlight Plug-In 4.0.50524.0 - Windows Media Player Plug-in Dynamic Link Library 3.0.2.629 - Windows Presentation Foundation 3.5.30729.1 - WPI Application Detector 1.0.0.1

I made quite a stupid mistake when installed two extensions for "mouse gestures" (All-in-One gestures and Mouse Gestures Redox). So closing the tab was set to the same gesture in both extensions. Of course as a result two tabs closed when I used this gesture.

So make sure which extensions are trying to make the same thing simultaneously and uninstall/disable/reconfigure one of them.