By default the icon is places in the tab bar. If you are using an extension that hides or modifies the tab bar you will need to temporarily enable the default tab bar in order to see and move the icon. If you are using the default tab bar and the icon isn't showing up then something unusual is going on and will need to be troubleshooted. Please make an issues here https://github.com/Aaron-P/ToggleResistFingerprinting/issues to discuss it.

I'm not really sure what you expected this extension to do, all it does is toggles on or off Firefox's built in "resist fingerprinting" setting which attempts to reduce the uniqueness of the data gleaned by fingerprinting methods. You can go here for a list of what that mode does: https://wiki.mozilla.org/Security/Fingerprinting#Technical_Details

The only sure way to prevent most fingerprinting techniques is to disable JavaScript on the sites you visit, either by setting javascript.enabled = false in about:config, or using an extension like NoScript or uMatrix; even then there are still some methods to get identifying data without JavaScript. Some other methods can be spoofed by doing things like faking canvas API call responses, but the efficacy of those methods is questionable and you can't spoof everything.

Even with this extension and multiple others, most user agents are going to be fairly identifiable. If it is that much of a concern to you then you should look at using something like the Tor browser bundle.

Since resist fingerprinting is a global setting I'm not sure how feasible a whitelist would be. It would essentially have to work that resist fingerprinting is completely disabled if a whitelisted site is open, then re-enabled if all whitelisted sites are closed, which may be pretty confusing for people who don't really understand that limitation. If there is interest in that I could work on it, please make an issue on the github repo if you want.