And exactly this is not working, maybe I was not clear enough in my first post. After removing it from the browser bar as you supposed and then restarting Firefox, the Webrep appears again in the browser bar.
Can you confirm this ?
I do apologise, I misunderstood your initial post. You are correct, after restarting FireFox it does indeed get re-added.
I have managed to come up with a work-around for you though
Right click on the WebRep symbol, there is an option called "Add-on bar", check it you should see a small bar appear at the bottom of the browser.
Right click on the WebRep symbol again, and click Customise... this time, instead of dragging it into the box, drag it on to the add-on bar and click Done.
Now when you restart the browser, you will find it stays there.
The good news is, you can right click the bar, and untick "Add-on bar" to hide the bar, and the WebRep icon stays hidden, and if you tick the Add-on bar again, the WebRep icon is shown again.