If you use Nightly Tester Tools to bump up the compatibility of an add-on to enable it on your installation of Firefox or Thunderbird, that does not guarantee that the add-on will work. Nightly Tester Tools is a tool for testing.
Add-ons are built on top of Firefox/Thunderbird code. When the Firefox/Thunderbird code changes (new version), that may break the add-on, or worse, the add-on may cause the new version of Firefox/Thunderbird not to work. This is why the whole “add-on compatibility” thing exists in the first place. Add-on authors can state which versions they know their add-on will work on. When you bump the compatibility, it doesn’t magically fix and bugs that may occur, when trying to use that add-on on a new version of Firefox/Thunderbird. You run the risk of breaking your installation of Firefox/Thunderbird.
For some add-ons, there may not appear to be a problem; but there’s no guarantee, because it hasn’t been tested. That’s the point.
The same applies when setting extensions.checkCompatibility to false in about:config.
Of course, the answer to that has always been the same – with or without NTT, as people were bumping extension versions a long while before – is that a user who takes the advice given frequently in the forums, and thinks a little first, has a separate ‘Test’ profile into which they install every extension (bumped or not) and check to see if a problem occurs.
As we know it is the Profile that gets corrupted in the event of incompatibility.
That’s why I think users should be advised to test any and every extension first, since a combination of certain (on their own, perfectly compatible with the program version) extensions in the same profile will cause a problem that didn’t arise before they ended up co-existing. And no extension developer can possibly test that.