The plugin icon isn't keeping its position in firefox for me at all though. I can move to where i want, then when close firefox and restart firefox the plugin's icon is right back to the default location which is to the right of the bookmarks icon on the toolbar..
Stays for me, where are you trying to place it ?
Why not try changing the firefox layout, as you can see from my image (I have a minimalist layout to give more space), I don't have the menu bar displayed - you can get it using the alt key if you need it. I don't know if that may help or not or if you are already doing that.
I also use the Nuvola theme, which is a smaller compact theme.