Do a clean (re)install:

• Download a fresh Firefox copy from http://www.mozilla.com/firefox/all.html and save the file to the desktop.
• Uninstall your current Firefox version and remove the Firefox program folder before installing that copy of the Firefox installer.
• Don't remove personal data when uninstalling.
• It is important to delete the Firefox program folder to remove all the files and make sure that there are no problems with files that were leftover after uninstalling.

Your bookmarks and other profile data are stored elsewhere (not in the Firefox program folder) and won't be affected by a reinstall, but make sure that you do not select to remove personal data if you uninstall Firefox.

You can initially skip the step to create a new profile, that may not necessary for this issue.

See http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Clean_reinstall

Thanks for the response.

This was going to be my next step, but it just tried it after getting this and it started working again.

The problem lasted a few hours.

No idea why it stopped or started again. Maybe it was some other Windows service or process interfering with it.

thanks again though. -paul

There is an easier solution. Simply close all you Firefox windows. Then re-open Firefox.

The issue seems to be related to memory. Re-starting Firefox clean the memory.

Sorry, doesn't fix the issue with Firefox 12.0. Something much more insidious is involved this time.

Nope, completely broken with Firefux 12.0. Hey, guys, how about a bit of automated regression testing here, please? Chrome and IE are just too rancid to even be considered.

Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of the extensions or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance/Themes).

• Don't make any changes on the Safe mode start window.
• https://support.mozilla.org/kb/Safe+Mode

• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

In my case this behaviour was caused by an addon (Tab Mix Plus).

• Deactivating (addon) > Restart (FF) > Acitvating (addon) > Restart (FF)

brought the solution.

For me the problem was caused by the Torbutton add-on.

I have read that there are about 5 or 6 other add-ons that also inhibit Drag and Drop functionality, not just the Torbutton add-on. The list is somewhere in the main Firefox support section (but it's tough to find the answer there twice).

The generic add-on troubleshooting page is at:

https://support.mozilla.org/en-US/kb/Troubleshooting-extensions-themes#w_disable-all-extensions

@perspectoff

Thanks for that. After disabling the Torbutton addon drag n drop tab reordering functionality was restored.

This issue has had a bug ticket created at trac.torproject.org: http://bit.ly/KuDqnu

@perspectoff

Thank you. Disabling Torbutton fixed it for me too. Cheers!

How is it possible that a plugin can be made responsible for disabling such a basic feature as dnd? In my case it had to do with a Unity plugin (Ubuntu) that afaik I didn't even install myself, but came 'free' with an upgrade. Please tell me how to prevent this kind of annoying behaviour in the future.

By the way, I found this out because dnd stopped working on rearranging bookmarks as well...