I've finally come up with the magic bit of CSS to work around this:
hbox#browser { margin-right: -1px !important; }
(to anyone reading this that isn't sure what to do with that, copy/paste it into ~/.mozilla/firefox/<your profile>/chrome/userChrome.css)
This may be not be the absolute most correct solution (I think some other element has a border/margin/padding of 1 where it should be 0, but I couldn't track it down), but it fixes the problem for me perfectly.
Woohoo!
I've finally come up with the magic bit of CSS to work around this:
hbox#browser { margin-right: -1px !important; }
(to anyone reading this that isn't sure what to do with that, copy/paste it into ~/.mozilla/ firefox/ <your profile> /chrome/ userChrome. css)
This may be not be the absolute most correct solution (I think some other element has a border/ margin/ padding of 1 where it should be 0, but I couldn't track it down), but it fixes the problem for me perfectly.