Rated 3 out of 5 stars

Seems to do what it says, although the links it generates didn't load properly in Google Groups (I just got redirected back to the Google Groups homepage). I'll blame Google Groups for that though. :)

Just a heads up that the overlay seems a bit broken in current nightly builds. The "Google Group Link" appears on top of the "From" header.

Rated 5 out of 5 stars

This addon does exactly what it says: it shows a clickable link to Google Groups where the message can be read.

I'm not sure what it "requires" Mnenhy or Enigmail for, as I have neither of them and it still works smoothly.

For bluebloy56: the link is in the email header information above the message content, where one can read the expediter email address, the message date... It shows up only if you don't pack it on a single line (using the arrow on the left of the subject).

Rated 1 out of 5 stars

"Displays a link to the Google groups url of a newsgroup post, and an option to copy it." Oh really. And just WHERE does it "display" that link and option to copy? "Requires Mnenhy". Oh really? And just WHICH header am I supposed to configure in Mnenhy to show your special header? I look at all the link-type headers and nothing popped as one that your add-on added to the list. Context menu on header didn't show any entries related to this add-on. Context menu on any of the displayed headers didn't show entries for this add-on. Context menu for the message view pane didn't show any entries for this add-on. Amazing how many developers never consider that they are exposing their wares to someone ELSE but don't give a clue on how to use their product. Yep, you guessed it: UNINSTALLED.

There's no need to get angry, I didn't ask you to install it. I just made this extension for myself and decided to share it with you if you want.
To answer your questions, it should be right there in the headers pane. If it doesn't, it's either a bug in my software or not a newsgroup post (maybe a mirrored mailing list e-mail instead?).
If it's a bug, feel free to fix it and send me a patch.