I am sorry, that NelliTab is not working anymore.

Please note in Firefox 66 Beta there is e.g. a regex parsing bug in main configuration file which causes the issue you described above, that most extensions marked as "invalid" and already installed extension do not work anymore. I had the same issue with my Firefox Developer Edition.

https://bugzilla.mozilla.org/show_bug.cgi?id=1522209
https://bugzilla.mozilla.org/show_bug.cgi?id=1523159

I can only recommend to not use any Beta version.

But of course i cannot eliminate that you hit another bug in NelliTab, but if the bug occurs in Firefox Stable as well please let me know, i try to fix it asap.