Char

Tested with Firefox 35 with clean profile, I do not experience any problem. 

It could be platform (OS) related, although the code has no platform specific part.

You can - open the browser console (ctrl + shift + j), copy the contents, paste into a text editor, find the lines that contains tabgroupsmenu, then paste those lines...

Never mind with the browser console, I can reproduce the case with firefox in Arch linux.

Seems so. No problem on Windows. And only the first window is affected, if you open another window the addon works fine. It seems there is a bug with eager...

Yes, I have disabled the newer versions until I have the time (motivation) to rewrite the addon.

Press Ctrl+Shift+J (open the browser console), copy the contents, and paste it here. Don't forget to scan and remove for private info (if any) in the console output.

This extension does not modify tab order. And I have never experienced this kind of issue before.

I do not see any error of the addon. I suggest that you temporarily install an older version of the addon.

From here https://addons.mozilla.org/en-US/firefox/addon/tabgroups-menu/versions/