When you download an add-on for Thunderbird, a pop-up displays instructions for installing the add-on. The third instruction needs to be updated as follows: Click the gear icon in the top right corner and select "Install Add-on From File". Select the file you downloaded and click "OK".
This needs to be fixed urgently. It is not at all obvious how to install a downloaded add-on, and the instructions as they currently read do not work. Many users will conclude that it is impossible to install downloaded add-ons and give up. However, updating the instructions as per firstname.lastname@example.org's suggestions is not appropriate, because the icon looks different on different operating systems. Apparently it looks like a gear on Windows, but on GNU/Linux it looks like a set of sliders. The instructions would need to refer to this button in a way which is appropriate for all its incarnations. Perhaps a better solution would be to fix the appearance of this button, or to replace or augment it with a text label. The problems with the instruction and inconsistency in the user interface are covered in more detail at <http://thunderbirdtweaks.blogspot.com/2011/08/xpi-files-or-how-to-install-add-on-in.html>
My suggestion: If browsing in Thunderbird, no instructions are necessary and the add-on should install via Add to Thunderbird. If browsing outside of Thunderbird, the third step of the instructions should be: 'From the options button next to the add-on search field, select "Install Add-on From File..." and locate the downloaded add-on.'
Um, how can this not be fixed over four months after it was filed? It's easy to fix -- just change some language in the install pop-up -- and it's affecting TB users every single day. I just got a complaint from the user of one of my add-ons that he couldn't figure out from the instructions how to install it. Please fix!