Current ExQuilla releases
You may download the current revision of ExQuilla from this links.
To install, download and save this file. Then from Thunderbird, open Tools/Add-ons, click on the Addon tools button and select "Install Add-on From File". Install the downloaded file, then restart Thunderbird.
Alternatively, just go to the addons manager in Thunderbird, search for ExQuilla, and install it from there.