Do you have more than one browser installed ? If you are using XP/Vista you will probably have IE installed by default, as well as Firefox by choice.
In that case, I think the problem may be that Windows does not know, or has forgotten, which browser is your default choice and needs to be reminded.
Two things to try.
1) go start>control panel>add or remove programs. In there, click on "set program access and defaults" and then expand the "custom" page and ensure that FF is set as default or move the tick mark to the appropriate radio button, clicking apply/ok.
That should be sufficient,...if not try plan 2
2) Open Internet Explorer, and in the options field set it as your default browser......reboot and then reset FF as your default.
Do either of these work for you ?