Sorry you're having trouble with it. Though v3 is a small subset of the v2 (because it's a rewrite from the ground up) it does work for the basic functionality. v2 will no longer work with future versions of FF as XUL based plugins are being phased out.

One thing you might be running into is Content Security Policy which you can see by bringing up the console (via F12). For example on addons.vaultpit.io you'll see something like: "Content Security Policy: The page’s settings blocked the loading of a resource at" in your console. This is not something I'm able to do anything about at this time and the plugin will not work on sites like that. To my knowledge addons.vaultpit.io is the only site I've seen this at.

At any rate, a few people have chosen to stick with v2 for now, if I find the time to continue development on it I will.

Also, if you want to check on bugs/features you can do that on github. If there isn't already an issue raised there that's what you are experiencing, opening a ticket there will let the developer(s) track the issue towards resolution.