WARNING: Performing the below action can result in Miramar becoming unstable or unresponsive due to an incompatible add-on. Do so at your own risk.

Since Nightly Tester Tools hasn’t been made compatible yet with Miramar Alpha 2 I had to find a manual way to force add-ons to work. This can be done via Thunderbird’s Config Editor (Tools >> Options… Advanced and click Config Editor button in the Advanced Configuration section).

1. Click the I’ll Be careful, I promise! button
2. Right-click in an empty area
3. Select  New then Boolean
4. In the preference name box enter: extensions.checkCompatibility.3.3a
5. Select: false

If this has been done correctly, a warning message should be appearing in Miramar’s add-on manager and any add-on will be installed. I imagine that when Alpha 3 is released this will have to tweaked again creating a new preference ending in 3.4a