Option 1 (recommended)
Just go to the addons page in Thunderbird, search for ExQuilla, and install it from there.
1. Go to the addons page in Thunderbird.
2. Click on Extensions on the left.
3. Click into the search bar at the top right, enter "exquilla" and hit Enter.
4. In the page that opens in a new tab, next to "ExQuilla", click [Add to Thunderbird].
5. Confirm the installation.
6. Confirm the restart of Thunderbird.
7. You have installed ExQuilla. Continue with the setup.
Option 2 (Manual download)
1. Right-click on the XPI download from our homepage , click "Save Link As..." from the context menu, and and save the file on your disk. (If you just click on the link, Firefox will think it's a Firefox extension and try to install it in Firefox, and that will fail.)
2. Open the Thunderbird addons page (see steps 1 and 2 of Option 1).
3. Click on the Addon tools button and select "Install Add-on From File" from the dropdown menu.
4. Select the file you saved in step 1.
5. Confirm installation, then restart Thunderbird (see steps 5 to 7 of Option 1).