You can configure Firefox to have no internal decorations.
For that you should create an own profile so it won't mess with your other browsing windows and then disable all sidebars etc. in the View menu.
If you want to have even less, like no Window decorations have a look at this Question over at Super User:
The basic Idea is that you create a Chrome Shortcut that opens in its own window.
The Prism Plugin for Firefox (something I have used before) is no longer available but if you are willing to install an old version of FF only for this purpose thats a solid working solution, even more so if you only have one site to display and don't use FF for your regular browsing.
Depending on what that site requires, it might be easiest to write yourself a small program where you can control the window decorations programatically (This is what I do today).
Windows that don't have decorations can't be moved by clicking on said missing decorations so to move it you have to right click the window in the task bar and select "Move" (or so, "Verschieben" in German) then you can move it with the arrow keys.