Rated 5 out of 5 stars
I was concerned by the previous review saying the addon no longer works in latest version. In making a limited test of a very old addon, checkCompatability seems to do exactly what it aims to (prevent FF from disabling plugins due only to the version number).
In v40.0.3 I tested it with an arbitrary older plugin: Custom Google Search v1.1.2. I was able to install the plugin, and on restarting, the plugin's custom setup screen appeared and allowed me normal interaction (it is 6 years old, so I couldn't do a "live" test with the current google API). Upon disabling checkCompatibility, the old plugin changed from the yellow "could not be verified" message to the red "has been disabled" one. On re-starting cC, the google plugin was instantly functional again.
The nature of "incompatible" plugins makes it probably impossible to perform a conclusive test, but there is certainly not a complete loss of function due to the recent major upgrade. Sadly, it's probable that any major FF revision will alter enough code that some older plugins will become genuinely incompatible. But if that's the case, we shouldn't give checkCompatibility 1-star reviews, if it still does its job of preventing the strict disabling of plugins marked as deprecated.