Thank you :-)

About middle-mouse click I will look what I can in 0.7

Edit:
middle-mouse click is now available in 0.7

Edit2:
It should be work on 0.7.x
I don't know, may be a OS or mouse driver setting is priority on the webextension.

(please add a black line and an 'Edit:' tag when you change your comment, mozilla send a notification when a user put stars, but send nothing when a comment is added or modified, then some time I check for new/modified comments, I has almost missed your change)

Edit3:
I think I have misunderstood, I have added middle-mouse click to open feeds (in the background), but what you want is middle-mouse click to open item (in the background). I will add that on the next version (0.7.7, I guess)

Edit4:
middle-mouse click for items articles is now available in 0.7.7