Thanks guys, but I cannot find exactly your suggested paths.
Neal I can find: lxdecc > Personal tab > Set Preferred Applications. and that sort of works but it seems to be overwritten by thunderbird somehow. I.e. When I open Firefox it is the default, but when I connect to it from an address in Tb it is no longer the default.
Leiche: your path does not exist in my installation of Firefox 8.
Firefox -> Edit -> Preferences -> Advanced -> General ->
Does not have the option "Set Firefox as default browser.", only "Always check to see if Firefox is the default browser on startup"
Whatever I do I don't seem to get a permanent solution. N.B. this has only happened since upgrading to 8.0.